Alan Dale, a New Zealander, has played several American characters, including a Vice-President of the United States in 24, the patriarch of the Meade family in Ugly Betty, the devious patriarch Caleb Nichol in The O.C.. He seems to be the guy you break out for evil American roles.
Cary Elwes: This Brit donned an American accent to play Apollo 11 astronaut Michael Collins (though ironically, the real Collins was born in Rome, and raised as an army brat in various US towns), while Canadian Dave Foley faked a southern accent to play Apollo 12 moon-walker Al Bean in From the Earth to the Moon. Cary Elwes also used a passable southern American accent in Kiss The Girls. Sadly, when his accent changed to a more "standard" American when he was revealed as the villain, he couldn't keep his real British accent from slipping through. He also played an American agent in season 9 of The X-Files.
Benny Hill: He tried to affect a New York accent in a few sketches playing an Archie Bunker-like character. He didn't even come close.
William Shatner: He's from Montreal and he appeared in Star Trek and Boston Legal. Kirk is born in America, Iowa, and he's born there during a time of "United Earth." So national boundaries don't exist.
Anthony Sullivan, one British but US-based infomercial host, is infamous for his mangling of words like "potatoes" and "forty five" to sound like the American pronunciations. Apparently he does this to appeal to the least intelligent demographic
Tammin Sursok: This actress (of Australia soap opera Home and Away) was cast as Colleen Carlton on American soap opera The Young and the Restless which launched her North American career. She has been in various movies and tv shows since then using an American accent. Most recently, as the blind Jenna on Pretty Little Liars. Although having an Australian accent, the actress was actually born in South Africa (as was her co-star Sasha Pieterse, although unlike Tammin she was brought up in the US).
She also played an American super model in the final season of Hannah Montana. For the most part, she managed to sound pretty convincing, but she seemed to have trouble with certain words and sounds and you could tell she wasn't really from the states.
Tracy Ullman, one British comedian, used a number of different American accents on her various sketch shows. She said that most British actors tend to sound very nasal when doing an American accent.
Jake Weber on American Gothic and Medium does an impeccable American accent. You'd really never think He was from England.
Examples listing shows
British TV detective series, such as Foyle's War, Sherlock Holmes, etc., have American characters turn up every so often. You can usually tell when a British actor is doing a Fake American, because he is SPEAKING FIVE DECIBELS LOUDER THAN ANYONE ELSE IN THE SCENE. Also, the minor American characters in these shows tend to be pushy and obnoxious, sometimes ludicrously so.
In Monty Python's The Meaning of Life, John Cleese, as the Grim Reaper, collects four people from a dinner party. When the American, played by real American Terry Gilliam, objects, Death tells him off: "Shut up, you American! You Americans, all you do is talk, and talk, and say 'let me tell you something' and 'I just wanna say'. Well, you're dead now, so shut up!"
30 Rock: An unusual version of this was seen on this show where British actress Emily Mortimer played a character who was ultimately revealed to be an American posing as a Brit. Naturally, her character's "fake" British accent was completely flawless and she only did one line with an American accent.
Alphas: British actor Ryan Cartwright adopts an American accent.
In-show, Angel is a Fake American (using David Boreanaz's own accent) from Ireland.
Gwen's actress, Alexa Davalos, is French.
Are You Being Served?: In one episode, Mrs. Slocombe's American uncle is played by a Brit with a very unconvincing accent.
A very unconvincing New York accent. And he is shown wearing a Stetson hat!
Arrow: Brings us Stephen Amell (Canadian) as Oliver Queen, Paul Blackthorne (British) as Quentin Lance, Jessica De Gouw (Autsralian) as Helena Bertinelli, Alex Kingston (British) as Dinah Lance and John Barrowman (Scottish but American-raised) as Malcolm Merlyn.
As Time Goes By: Had an arc where Lionel and Jean work on an American miniseries. The producer they associate with has an American accent that's just as bad as the miniseries.
"yrr in the shaht!"
Band of Brothers: This show is absolutely stuffed with British and other non-American actors, including but not limited to:
Damian Lewis (Winters)
Shane Taylor (Doc Roe)
Peter Youngblood Hills (Shifty Powers) - South African
Battlestar Galactica (Reimagined): Jamie Bamber (a native Londoner) played Lee "Apollo" Adama with an American accent, though his character isn't identified as such since the series is set several hundred thousand years before "Britain," "America," or indeed "the English language" were things. The idea was to make the relationship with Edward James Olmos more believable, in combination with Olmos wearing blue contacts and Bamber dying his blond hair dark.
And the reason he can do this? Because his father is an American, which automatically grants him US citizenship despite having been born and raised overseas, thus making him a subversion of this trope rather than a straight example.
Better Off Ted: Australian Portia DeRossi as American Veronica in this show.
Her accent is completely convincing until she says the word "anything".
Bionic Woman: Michelle Ryan who played Jaime Sommers in this 2007 TV series fakes it. But this is also subverted in one episode where Jaime goes undercover at a college as a British student, with Michelle using her own accent. The episode also stretches the subversion to include a sequence where she continues to use the fake/real British accent while communicating with a fellow agent, who wonders why she's continuing to use the accent but admits he finds it sexy.
Boardwalk Empire: Real Life gangsters Al Capone and Meyer Lansky are played respectively by Stephen Graham and Anatol Yusef, both British.
Boston Legal: In Season 4, the English actress Saffron Burrows joined the show as lawyer Lorraine Weller, with a really bad fake American accent. After a few episodes (presumably after the producers realized the tremendous mistake they had made by casting her as an American), they used a hastily concocted plot twist (she was in hiding from her past life as a high-class British madam) that enabled her to revert to her natural speech, and the horrible American accent was never heard again.
Breaking Bad: Laura Fraser does an impeccable American accent as Lydia. Even her co-stars had no idea that she is really Scottish.
Brothers and Sisters: Matthew Rhys (Welsh) and Rachel Griffiths (Australian) put on a very convincing American accent.
Buffy the Vampire Slayer: Double Parody: James Marsters (an American) playing (British vampire) Spike attempting a bad American accent ("No, sirr, eye'm a frrend o' Xanderrrs.")
British Gabrielle Anwar normally does a pretty darn good job of pretending to be an Irishwoman pretending to be American. Her accent works very well. Only rarely does it slip, usually when overpronouncing arrs she'd normally leave out. This started with the second episode, after the audiences cringed at Anwar's attempt at being a Fake Irish in the pilot. The in-universe reason is that she can't "go around speaking like a bloody leprechaun" in Miami.
One episode has the New Zealander Lucy Lawless faking an American accent.
Californication: Natascha McElhone, born in London and raised in Brighton, plays Hank Moody's sassy, East Coast American for-all-intents-and-purposes-wife/ex-wife/lost love on this show.
Castle: Strangely, both Castle and Beckett's actors, Nathan Fillion and Stana Katic, respectively, are Canadians.
Chuck: Yvonne Strahovski (Australian) plays a CIA agent. She uses her native accent in one of the episodes when she's posing as an Australian scientist.
The Closer: American Kyra Sedgwick (a New York Blue Blood if there ever was one) fakes the Southernspecifically Georgia (it's good enough!) accent of her character Brenda Johnson.
CSI: Crime Scene Investigation: Louise Lombard played Sofia Curtis on this show, and did such a poor job trying to sound American that it turned into a What the Hell Is That Accent? situation.
Damages: Rose Byrne (an Aussie) plays Ellen Parsons, a young American lawyer, with a great American accent.
Deadwood: The creators of this show were unsure whether Ian McShane could completely pull off an American accent for his role as Al Swearengen. They added English ancestry to his past, something untrue of the real Swearengen, to explain any slips.
Paula Malcomson, who plays Trixie is from Northern Ireland but manages very well.
Immigrant backgrounds are a good way to cover up any accent problems in the plot and are entirely plausible, historically. Hence Polish Izabella Miko and South African Alice Krige play transient prostitute characters in the third season.
Defiance: Tony Curran (Scottish), Jaime Murray (English), Mia Kirshner (Canadian), Stephanie Leonidas (English) and Grant Bowler (New Zealand-born Australian) all play Americans with very good accents, especially Curran and Murray. At the same time, Curran and Murray's characters are Human Aliens who have adopted American English as their primary language.
"American" Companion Peri Brown is played by British Nicola Bryant. Bryant has confirmed in interviews that when she joined the series an attempt was made to hide the fact she was British, to the point where she was asked to stay "in character" as an American even for TV interviews. The charade lasted only a few months before she was allowed to be a Brit again off-camera.
In "Dalek", the American characters are played by Kiwi Anna-Louise Plowman, Canadian Nigel Whitney, and British Steven Beckingham. Corey Johnson (who plays the Big Bad) was born in New Orleans, but much of his accent sounds strained and over-precise at times; it's obvious he's heavily trained and spends a lot of time in the UK.
"Daleks in Manhattan"/"Evolution of the Daleks" had British Miranda Raison and Ghanaian-born British-raised Hugh Quarshie as guests of that story.
As well as the rarely playing a british guy, Andrew Garfield doing an Oklahoma accent.
Brits Mark Sheppard and William Morgan Sheppard play Canton Delaware in "The Imposssible Astronaut"/"Day of the Moon". Other guests include Nigerian Chukwudi Iwuji, British Mark Griffin and Canadian Kerry Shale.
In "The Sontaran Stratagem"/"The Poison Sky", British actor Ryan Sampson plays Luke Rattigan.
In The Tomb of the Cybermen, Vienna-born George Roubicek and Welshman Clive Merrison play Captain Hopper and Jim Callum. Their accents are quite awful.
In The Chase, the Daleks chase the First Doctor and his companions through a New York skyscraper, where a tour guide (Noo Yauwk) and a tourist (Allabayama) are entertainingly fake. William Hartnell's Old West adventure, The Gunfighters, is full of the same.
Don't forget Jack Harkness, played by Glaswegian John Barrowman, who does have an American accent in real life (except when talking to his parents). Harkness is a Fake American In-Universe as well, being from another planet in the 51st century but posing as an American officer in World War II. He does frequently show his British assimilation by yelling "Oi!" at people.
Dichen Lachman puts on an American accent when she is Sierra (and for various imprinted identities) and uses her natural Australian accent for Sierra's true identity, Priya Tsestang.
There's British actor Mark Sheppard as American FBI Agent Graham Tanaka.
The Dresden Files: The TV series featured Fake American Paul Blackthorne (who was born in Shropshire, England and who grew up on British military bases in Germany and in England) as the very American wizard-private investigator Harry Dresden. The show also starred Fake Brit Terrence Mann (who was born in Ashland, Kentucky and who grew up in Largo, Florida) as ghost-with-a-Teutonic-name-and-a-British-accent Hrothbert of Bainbridge. Bainbridge, by the way, is a real town in North Yorkshire.
Dr. Quinn, Medicine Woman: Jane Seymour has played many American charactersnote and became an American citizen in 2005, most notably this show.
In fact, as Dr. Quinn, she spoke with an old-world Boston accent which is similar to a British accent, at least as far as American accents go.
Eleventh Hour: Rufus Sewell seems to be doing an American Standard accent.
Eli Stone: The character is supposed to be an American lawyer, who is played by British actor Jonny Lee Miller, who also faked a Scottish accent in Trainspotting, but that's a different trope altogether.
Eureka: James Callis as Dr. Grant. Since Grant is also a time-traveler, he has to affect an American accent from the 1940s. The accent is really terrible, although some of that is attributable to the occasionally narmtastic lines he has to deliver.
Farscape: Claudia Black has a pretty feeble accent when she plays Crichton inhabiting Aeryn's body. Her body language, though? Pure Ben Browder.
In another episode where John and Chiana are trapped inside a video game Claudia Black plays a princess and affects a ridiculous Southern accent (with a lisp for some reason). Since it's a fairly silly episode anyways, it's a bit more forgivable
Gigi Edgley (who played Chiana) drifted between an American accent and her native Australian accent, depending on the episode. Anthony Simcoe (who played D'Argo) did a better job, though
The inversion from the same series, where Ben Browder tries to sound vaguely British when Crichton is masquerading as a Peacekeeper officer, is really no better either. (Browder is from Tennessee and simply used his real accent when playing Crichton)
Of course, considering whereFarscape was filmed, most of the regular Peacekeepers were Fake Brits.
During the 20-second Farscape parody scene in the 200th episode of Stargate SG-1, Canadian Michael Shanks is similarly feeble at Crichton's distinctly Southern accent. He says 7 words, but is plenty horrible anyhow. This may have been intentional, given the over-the-top nature of the numerous other parodies in the episode. (Of course, Shanks' regular character in the show is also American, but he still sounds Canadian....)
Fawlty Towers: In the episode "Waldorf Salad", the ugly American, Mr. Hamilton, is played by the Canadian actor Bruce Boa, with an audible Canadian accent.
Frasier: Parodied by Daphne only being able to say one word like an American. ("Sure.")
Jane Leeves, who plays Daphne, was born in Essex and brought up in Sussex, but puts on a Mancunian (i.e. from Manchester) accent throughout the series. See Fake Brit for more details.
A later episode shows that Daphne has gotten "better" at her fake American accent when she holds an entire phone conversation with it. Frasier compares her deep-voiced attempt to the voice of a drag queen.
The Daphne example becomes doubly-amusing once you learn that John Mahoney, who played Frasier's all-American dad Martin, is an ACTUAL Mancunian (though he spent decades in the US before this show began).
Fringe: Set mainly in Massachusetts, stars two Australians and a Canadian in the lead roles.
The Full Monty: Mark Addy, the 'fat bastard', did a fake American accent for four years on Still Standing.
The same one he did while playing Fred Flintstone, basically. Of course, in that case, he was supposed to sound like a cartoon...
The Gates: Paul Blackthorne also plays an American vampire in several episodes.
Generation Kill: Swedish actor Alexander Skarsgard plays US Marine Brad Colbert. His accent isn't entirely accurate at times, but it lends itself toward Colbert's status as Bravo Company's "Iceman" and Cultured Warrior.
Likewise, in True Blood, Skarsgard sports an American accent to play Eric Northman, a vampire who had lived in America for many years. British actor Stephen Moyer and Canadian-New Zealand actress Anna Paquin also both sport southern American accents to play Bill and Sookie, respectively. Though Moyer's had a few instances where his real accent slipped through.
Also, Australian actor Ryan Kwanten, who plays Sookie's brother Jason. However, Sam Trammell (who plays — naturally — Sam) presumably uses his native accent, being a Louisiana native.
Gossip Girl: Ed Westwick is a Brit who plays American Chuck Bass.
Parodied early in the second season, when Chuck fakes the (very bad) English accent of the aristocrat who's romancing Blair (instead of using his own natural accent).
Hannibal: British Hugh Dancy as Will Graham. The series also has multiple Canadian actors playing Americans. Caroline Dhavernas (Dr. Alana Bloom) gets extra credit since she grew up speaking French.
Haven has Adam Copeland, who makes no attempt to hide his Canadian accent despite playing a Maine native. Lucas Bryant does a bit better (though there is a hilarious clip in the season 4 blooper reel where he is unable to say "resources" without sounding Canadian, and gets increasingly upset with himself as he tries and fails.)
Hawaii Five-0 (2010 version): Aussie Alex O'Loughlin as Steve McGarrett.
Homeland: Has three: Nicholas Brody is played by Damien Lewis, David Estes is played by David Harewood, and Peter Quinn is played by Rupert Friend, all of whom are British.
House: Hugh Laurie as Dr. Gregory House. When he auditioned for the part, his accent was disguised so well one of the producers of the series thought he was an American from the same region as the character. Maybe it had to do something with the fact that his American voice sounds really, really creepy and ominous.
People who'd seen him in the live action film of 101 Dalmatians (he plays one of Cruella's henchmen) have heard his native accent.
Bryan Singer actually held him up to the other producers as a shining example of what a 'real American actor' was. It's quite jarring to hear Laurie speak in his native British accent these days. It takes quite a while to realize that the Blackadder Laurie and the House Laurie were the same Laurie* For those who don't follow British comedy, he also appears with full English accent in the Friends episode "The One with Ross's Wedding" (season 4, episode 23). Laurie's character spends most of his lines berating Rachel during her last-minute flight to London.
Laurie does however do his 'George (Blackadder) voice' in an episode where he tries to convince someone over the phone that he is English.
Lampshaded in an episode of House called "Poison", in which Chase uses a horrible fake American accent (Southern, of course) to trick a patient's mother. When he demonstrates the accent, House replies, "You fooled her with that?"
He also does a reasonably good American accent in season six when House told him women were only interested in him for his looks, and challenged him to try speed-dating without the accent or mentioning his profession.
A recurring skit on A Bit of Fry and Laurie had Laurie and his co-star Stephen Fry both playing overbearing American tycoons; which typically consisted of horribly, and obviously, fake American accents, and cursing at high volume.
Laurie has released a blues album called Let Them Talk, on which he sings with an very twangy American accent.
The two times Hugh Laurie hosted Saturday Night Live zigzagged this trope. Most of the time, he spoke with his native British accent while other times, he spoke with an American-sounding accent that was more-or-less a toned-down version of the voice he uses when he plays Gregory House.
JAG: David James Elliott aka Harmon Rabb, Jr. in this show hails from Canada. However, he did become a U.S. Citizen during the show's run.
Jeeves and Wooster: So many people. Some of them are just talking British with an American accent.
The Last Days Of Lehman Brothers: In this English TV movie, James Bolam puts on an atrocious "Southern" accent.
Last Resort: Another fake American Naval officer, Lt. Cmdr. Sam Kendal of the USS Colorado in this show is played by British-Canadian actor Scott Speedman.
Law & Order: Linus Roache's sort-of New York accent. It has improved since he started...
LOST: Evangeline Lilly, a Canadian, plays Kate, who is from Iowa. In an early flashback, before we knew much about her backstory, Kate mentioned being from Canada, but she turned out to be lying. Since then, Canada has become a Running Gag, in that every time Canada is mentioned, the character who does so is lying about something.
Mash: Intra-American example: David Ogden Stiers from central Illinois played Bostonian Major Winchester on this show with a strong Brahmin accent. Roger Ebert, who went to high school with Stiers in Urbana, IL, once commented that he sounded "like that" even back then.
The Mentalist: Simon Baker (Australian) and Owain Yeoman (Welsh). One episode has Jane being followed by a man who claims to be a government agent, whom Jane quickly figures out to be British (the guy puts on a very House-like accent).
Monk has Canadian Jason Gray-Stanford playing American Randy Disher. Mark Sheppard also shows up in one episode playing the killer of the week and again putting on an American accent.
Monty Python's Flying Circus: In the second episode (the marriage counsellor sketch), John Cleese attempts (unsuccessfully) a cowboy drawl. (Significantly, the version of this sketch included in And Now For Something Completely Different, a Compilation Movie targeted at American audiences, replaced the cowboy character with the voice of God.) In the third episode, the "Bicycle Repair Man" sketch features deliberately exaggerated American accents.
John did this again in the "Attila The Hun Show" sketch. Since the joke was about putting a major historical character into an American-style sitcom, he naturally attempted a generic Midwest/California accent. It's... not very good.
Apparently all you have to do is add r's there and there.
Nope, that's just the Pythons. Even Terry Gilliam, who is American (he's from Minnesota), can't do a convincing American accent.
Chapman manages an accent that sounds fairly American in that clip, it's just that he can't seem to settle on one region: It's a weird mix of John Wayne (Iowa), JFK ("idear", Boston), and something vaguely Texan.
Michael Palin is possibly the worst here, though he has pretty stiff competition from both Terry Jones and Chapman, Mike doesn't even bother for most of his lines.
On the other hand, in the several Python performances where she used a Yank accent ("Scott of the Antarctic", etc), Carol Cleveland's pronunciation was flawless... being half-American probably was a contributing factor.
In that same sketch, Eric Idle plays an obviously-American film producer. His accent isn't awful, but he gives himself away when he over-corrects his "r"s.
Moonlight: Two of the main characters are played by Fake Americans Alex O'Loughlin (Australian) and Sophia Myles (British). In a subversion, Shannyn Sossamon, who is American plays a Fake American character (Coraline was born in pre-revolutionary France). In fact, most of the older vampires can be considered to be Fake American characters, especially the 700-year old Lola (played by O'Loughlin's then-girlfriend Holly Valance, also an Aussie).
Josef Kostan, played by the Jason Dohring (American), was originally supposed to be played by Rade erbedija (Croatian) and would've had a thick East-European accent.
MythBusters: Robert Lee, the narrator of this show and others, is a borderline example—he was born in England, but raised in America, and now lives in Australia. He does his voice-over with an American accent, but uses many, many Britishisms, such as "aluminium" or a person being "in hospital," to name just two.
Nashville: This series has no less than five Fake Americans, with British Sam Palladio as Gunnar, Australian Clare Bowen as Scarlett (who notably sports the thickest Southern US accent on the show), Canadians Lennon and Maisy Stella as Maddie and Daphne Conrad, and Dutch Michiel Huisman as Liam.
New Amsterdam: Nikolaj Coster Waldau plays John Amsterdam in this show and has a good American accent, despite the fact that both he and (technically) his character are Fake Americans: the character was born in the Netherlands and the actor is Danish. Oh, and John's parter Eva Marquez is played by a Brit, and therefore is also a Fake American.
Nip/Tuck: Stars Julian McMahon (who is from Australia) and Joely Richardson (from England) as Americans. In general, Richardson's American accent is much more accurate than McMahon's (except when she says the word "family").
NUMB3RS: In the Season 5 finale, James Callis, a Brit, plays cult leader Mason Duryea with a believable Southern drawl.
The Office (US): Speaking of Idris Elba, he played the dry but quite American Dunder-Mifflin executive Charles Miner on the American version of this show.
Poirot: Jaime Murray in the episode "Mystery of the Blue Train".
It's a little bit more complex than that. Most seasons there's a mix of Aussie and Kiwi amongst the cast, and there have been occasional Canadians (Kevin Duhaney as Ethan in Dino Thunder, a big percentage of the SPD cast), British (Samuell Benta as Will and Rhoda Montemayor as Rose in Operation Overdrive) and even a few Americans (Monica May as Z in SPD).
Rose was actually shown to be a university lecturer in London prior to becoming a ranger, so she averts the trope.
James Napier's horrible and incessant accent slipping inspired a drinking game circa Dino Thunder. This is especially noticeable when he has to say anything fast or lengthy (or both). The "Conner was attacked by giant lizard-things" speech in "Wave Goodbye" sounds downright COCKNEY as the Kiwi and American accents jockey for control.
Oddly, McGoohan actually was born in America. His character on the 1959-60 Danger Man series, John Drake, was in theory supposed to be an American working for NATO, but he basically sounded like McGoohan.
Pushing Daisies: Anna Friel, who is English, pulls off a very convincing American accent as Charlotte "Chuck" Charles.
Red Dwarf: The Cat was supposed to be portrayed as a flashy American type. Danny John-Jules had a decent accent, marked by occasional cat-like yowls; but once in a while, British usage would slip in. e.g., "What I don't understand is why he went through the trouble of having to use his kidney as a full-stop." (A full-stop is a period)
Then again, the Cat's race presumably learned English from documents on board the Dwarf which seemed to be staffed almost entirely by Brits.
It's mentioned in an episode that the cat race learnt English from Red Dwarf's large selection of American movies. However, when Cat is seen reading in the early series he does so by smell. He has learnt to read later on so it is presumably possible that he was taught to write by members of the crew, who would have used English terminology. Alternatively, he uses terms like 'full stop' because he has gotten used to using them.
One major exception being the Captain, who was an American played by real American Mac MacDonald (who has made a career of playing Americans on British TV).
Craig Ferguson, in one of his earliest TV acting roles, played Confidence in "Confidence & Paranoia", whom Lister described as sounding like "Bing Baxter, the American quiz show host". His smarmy quiz-show host accent is pretty good, which makes the fact that Ferguson later moved to the US and hosts a popular American late-night show where he doesn't bother to cover up his natural Scottish accent oddly hilarious.
In the same series, Robert Llewellyn who plays Kryten. Like the Cat, he is technically not an American (or human) but he speaks with an "American" sounding accent.
It's mentioned that Kryten learnt to speak with that accent due to watching Rebel Without a Cause and idolising James Dean, trying to be like him and as a result breaking his programming. The accent is much closer to a Canadian or northern US accent, something which the creators have noted.
One old episode featured John Candy in a parody of Midnight Cowboy with a bizarre, hooting accent that was evidently supposed to be Southern US. It was intentional though, as the character's brain-meltingly awful accent (where "y'all" came out as a bizarre, howling "yole") was mocked at least once in the sketch.
Lampshaded in an SCTV sketch lampooning Midnight Cowboy, in which Canadian John Candy affected a "Southern" accent so tortured that "y'all" came out "yole" and the entire effect was weirdly similar to yodeling.
Smallville: Contains many Canadian actors, as it is filmed in Canada, despite taking place in Kansas and all characters listed here are from Kansas.
Erica Durance (Lois Lane)
Kristin Laura Kreuk (Lana Lang)
Aaron Ashmore (Jimmy Olsen)
Laura Vandervoort (Kara)
Actually, Kara's from Krypton.
Sons Of Anarchy: Charlie Hunnam from Newcastle, England, plays the American biker Jax Teller in this show. The accent he uses is a classic example of an English person overdoing an American accent, but at least works for the purpose of carrying the story. He tends to completely overdo the accent when the character is angry, leading to Large Ham moments.
Spooks: Has various CIA agents who generally sound half New York City, half Midwestern, and thoroughly Jerk Ass.
The later series have some dreadful accents (so bad even British people can tell). Most notably Irish-Australian actress, Genevieve O'Reilly as CIA operative, Sarah Caulfield.
Stargate Verse: Many characters are Canadians playing Americans, because a lot of shoots and almost all outdoor shoots are done in and around Vancouver.
Samantha Carter, played by English-Canadian actress Amanda Tapping. Most of the time there's no problem, but a few words in particular she pronounces in a more English or Canadian manner (For example, the word "assume", which she pronounces as "uh-SYOOM" while most Americans say "uh-SOOM"). We don't know where exactly the character's from, so maybe when she was a kid she lived somewhere that she would pick up a few "exotic" pronunciations?
It's particularly glaring in one scene where Carter comments on Rodney's and his sister's Canadian pronunciation of "sorry," then immediately says the word with the exact same pronunciation herself.
Dr. Daniel Jackson, played by Canadian Michael Shanks. Like the Carter example above, his accent is mostly fine, but every once in a while a "sore-y" or two slips out.
Particularly amusing is the one sentence he speaks while playing John Crichton in the 20-second Farscape parody in SG-1's 200th episode. He imitates the Southern accent of his co-star Ben Browder, who played Crichton originally, and the result is less than perfect. But since it's an over-the-top parody, that's okay.
To Be Fair, Keller is supposed to be from Chippewa Falls, WI — and a lot of us from the region really do have pretty much the same accent.
On the other hand, in Staite's other famous science fiction role as Kaylee Frye on Firefly, the Oklahoma-esque accent she seems to be attempting slips constantly.
Strike Back: Sullivan Stapleton, an Australian, plays the allegedly American Damien Scott in this Cinemax action show. For extra irony/mind screw points, his "British" counterpart Michael Stonebridge is played by American Philip Winchester.
Thunderbirds: Gerry Anderson made a conscious effort to make most of the characters American, in order to improve his chances of selling the show internationally. The voice actors who provided the performances were almost all British (the number of actual Americans who provide voices for the Anderson shows can literally be counted on one hand - David Holliday, David Healy, Robert Easton (Stingray) and Ed Bishop), and while the accents themselves are convincing, the dialogue is marked by word usages and slang exclusive to the UK.
Paul Blackthorne (British) appeared in a Christmas Episode of this show using a pretty bad American accent.
Jaime Murray (British) temporarily has an American accent after her character Helena Wells undergoes Laser-Guided Amnesia and is programmed to think she's an American schoolteacher.
The Wire: Has several non-US actors including Dominic West (who plays Jimmy McNulty; is from Sheffield, England), Idris Elba (Stringer Bell; London, England) and Aiden Gillen (Tommy Carcetti; Dublin, Ireland) who attempt American accents with varying degrees of success.
In the Season Two episode Stray Round, the show has an example of Lampshade Hanging when McNulty fakes a British accent while going undercover to expose a prostitution ring. His Brit accent is awful at first, but it improves (when the actor basically speaks in his natural British accent).
Idris Elba was so dedicated to his accent (and so good at it) that he used his Stringer Bell voice during promotional appearances in order to avoid disturbing Wire fans.
Without a Trace: Anthony LaPaglia (Jack Malone), Poppy Montgomery (Sam Spade), and Marianne Jean-Baptiste (Vivian Johnson). The first two are Aussies, the latter British.
Although LaPaglia and Montgomery have been putting on the American accent for so long that they scarcely sound Australian any more. In fact, Montgomery once said that her Australian accent has atrophied so much that when she attempts it with her family, they tell her to just give up.
Xena: Warrior Princess: Kiwi Lucy Lawless uses a flawless American accent, and it's startling to hear her native, higher-pitched accent in interviews.
Some of other regulars, such as Karl Urban (Caesar) and the late Kevin Smith (Ares) also can pull this off well. But there are plenty of other minor villains who...don't.
The characters in Hercules: The Legendary Journeys and Xena all use American accents; a particularly jarring example is Claudia Black as a minor character in both. For people used to her normal accent (which is already a strange conglomeration of Australian and English anyway), her attempts at speaking with an American accent are even more distressing.