Mazinger Z: Most of the female characters wore long hair: Sayaka, Misato, Erika, Hitomi, the Gamia sisters... Sayaka even wore Hair Decorations (a pink headband). Of course, it was considered a girl seemed more feminine like that. In Great Mazinger, Jun—and even a villainess like Marquis Janus—wore also long hair. It was kind of subverted in UFO Robo Grendizer, though: Hikaru—a feminine Yamato Nadeshiko—wore short hair, and her best friend Maria—a Tsundere tomboy—wore long, curly hair.
In Samurai High School, when Tsukiko decided to enter school as a boy, she considered cutting her hair but her brother said it'd not be necessary. Just holding it in a ponytail would be enough. He was right. Also, he wore a wig to pose as his sister.
In the The Sandman story "A Game of You", the transwoman Wanda has long red hair that, according to her friend Barbie, she is rather proud of. When she dies, her family, who never accepted her as a woman, have her buried with short hair as part of their erasure of her female identity.
In W.I.T.C.H., Cornelia has the longest hair of all the girls, and she does figure skating.
In most fanfics which feature a male character turning female, said character will automatically get long hair. Because hair length is clearly determined by the presence of two "X" chromosomes.
Films — Animated
Played straight with most female human characters from the Disney Animated Canon films, but subverted with Snow White, Cinderella, and Tiana, who all sport bobbed hair (most likely because the first two were both given hairstyles corresponing to thedecades their respective films were released in, and the third to match the time period her film takes place in), as well as Mulan and Rapunzel, who both start out with long hair, but inevitably have their hair cut short.
Films — Live-Action
The Disney Channel movie Motorcrossed had a girl cut her hair short to make her look identical to her twin brother.
The Milla Jovovich version of Joan of Arc has Joan annoyed that the soldiers aren't taking her seriously because she's a woman, so she hacks off her hair in the hope that she'll be considered as one of the men.
The biopic of the first female U.S. firefighter has said woman having to cut her hair short for health and safety reasons, and she complains that she looks like a boy now.
In The Brothers Grimm, one of the village children is mistaken for a boy because she had short hair while all the other little girls had flowing long hair.
In Showgirls one of the girls auditioning for the Vegas dance troupe has short hair and is told "I hope you have a good collection of wigs, sweetie". Cristal also has quite short hair (albeit just above the shoulders) but wears a long hairpiece whenever she's performing.
Fantine in Les Misérables is heartbroken to have to sell her hair — her crowning beauty, which falls all the way to her hips and is a beautiful blond color — but she does it without a second thought to earn money for her daughter, Cosette.
Della, in The Gift of the Magi, has hair falling almost to her knees. She has it cut, obviously, and frets that now she looks like a "Coney Island Chorus Girl".
In The Rape of the Lock, an assault on Belinda's hair is considered an assault on her person, and her beauty, though her hair is still about 90% intact.
One short children's book dealt with a girl lamenting over her shortened hair making her look like a boy after a haircut. The trope is Inverted when she meets a construction worker with long blond Compressed Hair who reminds her that hair length doesn't make the gender.
Inverted on a large scale in A Brother's Price, where short hair is feminine. Children and men grow their hair out long, and men usually keep theirs braided.
The opening scene in Terry Pratchett's Discworld novel Monstrous Regiment has Polly Perks clipping her hair short so she can pass as a boy and enlist in the army. The fact that she then takes the hair with her becomes a bit of a plot point.
Honor Harrington: Grayson as a whole feels this way; it stems from their conservative Christian theology. Notably, when Abigail Hearns, Miss Owens, becomes the first Grayson woman to join the Navy, she refuses to cut her hair, despite the inconvenience of maintaining her waist-length brown hair as a serving officer. Properly raised Grayson women do not cut their hair short, and it seems that you can take the girl off Grayson, but you can't take the Grayson out of the girl.
In Veronica Mars, the Veronica of flashbacks, when she was a fairly stereotypical high school girl, has long hair; present-day Veronica, having become a Guile Hero by way of Break the Cutie, favors a much shorter, more severe cut.
An episode of Chicago Hope had a subplot about a boy who had been raised as a girl. When s/he found out, s/he cut off all his long hair to look more like a boy.
In an episode of Charmed when the sisters cast a spell to turn one of them into a man, her hair becomes significantly shorter (and she gains a goatee) as part of her gender transformation.
Every single one of the girls in New Directions has long, flowing hair (with the occasional exception of Mercedes when she wears her hair natural) right up until the second season finale when Quinn, having lost her Prom King potential boyfriend, proceeds to fail at villainy as well and gets a cute short haircut from Santana and Brittany to make up for it.
Morgana, Guinevere and Morgause from Merlin (as well as most of the female guest stars) all have beautiful long, shiny, styled hair.
The Worst Witch TV series uses this trope to contrast Miss Drill with the rest of the characters. As the only non-witch member of staff she has short Spiky Hair and is always wearing sports clothes. The rest of the teachers and the girls all have long hair and all wear skirts and dresses.
The documentary Whose Hair Is It Anyway? examines this trope in relation to the use of hair extensions amongst modern girls. One woman spent £400 a month getting extensions put in and would be paranoid about leaving the house without them in. Presenter Jamelia admitted to spending 3 hours every morning getting her hair ready before school. They theorise that the desire for really long hair comes from the classic fairytale princess look.
Jamelia's travels take her to India where she discovers that long hair is the mark of a woman's beauty. Women sacrifice their hair to the temple as an act of faith, and the temple in turn sells the hair to extensions dealers. The money from that is used to feed the poor. Long hair is also highly valued in Russia, where Jamelia sees fresh hair being cut to make extensions.
The Survivor-like modeling series America's Next Top Model each season plays with this trope and Boyish Short Hair, as the almost always long-haired models have their hair cut, sometimes a great deal. At least once, this caused a contestant to quit the competition.
Inverted in Once Upon a Time where the most gentle and feminine of the women Mary Margaret has the shortest hair in the cast - and her Enchanted Forest counterpart Snow White (who was an Action Girl) had long hair. Likewise tough cookie Emma is probably the least feminine character and she has long blonde hair.
The WWE Divas are generally portrayed with long hair, bordering on Rapunzel Hair. As such they all fall under the category of Girly Bruiser. Some Divas have gone for shorter hairstyles such as Candice Michelle and Ashley Massaro but they usually end up putting extensions in after a couple of months. Alicia Fox changes her hair drastically sometimes with a shorter style but she tends to end up with long hair again. Molly Holly gave herself Power Hair to emphasise her prudish and unfeminine character and even had her head shaved at one point.
Female wrestlers will normally only have short hair if they are giving themselves a decidedly unfeminine gimmick. For example Roxxi in TNA was the Hardcore Knockout, Alpha Female is The Giant etc.
In El Goonish Shive, whenever a character changes genders, their hair usually gets longer or shorter to complement their new gender. Justified, as it's built into most of the forms they usually transform into, especially the Female Variant #5 form.
Most Rage Comics faces are drawn without hair and assumed to be male by default. When an author wants to indicate a character as female, they simply add shoulder-length hair (with a hair dec) to the corresponding rage face.
minus invokes this here — she is mistaken for a boy, and consequently grows her hair (previously a bowl cut) long and wavy, wears an ankle-length dress, adds a bow, pink, to her Idiot Hair, gives herself Fertile Feet, and makes a couple butterflies for good measure.
Homestuck's minimalist art style doesn't include sexual characteristics, so a lot of its female characters have long hair. Although given that three of the four human girls have short hair (the one exception being Jade's Rapunzel Hair), and all of them are relatively feminine, it's also a partial subversion. Additionally, among the trolls, a few of the boys have long hair, so...
Anvil, of Grrl Power, is dangerously susecptible to the trope. Word of God says that she keeps her hair very long due to some insecurity about her femininity, which in turn is due to her exceptional height and formidable musculature. However, this may not be a good choice for a superhero — and a brawler at that. She's smart enough to tie her hair up securely if she knows that a fight is coming up, but she doesn't always get the chance — and at least one opponent takes full advantage of this.
Iphigenie from Greek Ninja has very long hair and is one of the most feminine characters.
In Avatar: The Last Airbender, Katara, the first prominent female character, is the only girl in the core cast whose hair is long and worn down (if not loose, i.e. braided) at all times. Katara is also the most mothering, nurturing and feminine of the cast — in addition to being the Love Interest. Toph and Azula both have long hair that is almost always up, and Suki has short hair.
Inverted with Jessica and Emilie in Wheel Squad. Jessica has the longest hair of the duo and is so much of a tomboy that, when she entered a figure skating contest and some of her rivals said she had no chance for not being feminine, her friends weren't able to say she was.
In My Little Pony: Friendship Is Magic, Fluttershy and Rarity fit this trope to a T. They're the two girliest mane ponies, and they have long manes, Fluttershy's actually is long enough that part of it trails on the ground behind her. As a rule, even in background ponies, the mares have the longer manes.
In Winx Club, Bloom, Stella, and Flora have the long hair and are quite feminine.
Nuns entering a convent would cut their hair off as a sign of them giving up their femininity to serve God.
The ultimate expression of this trope: The Pioneer plaque, meant to show aliens what humans look like, depicts a man with short hair and a woman with long hair.
David Reimer is notable for having originally been assigned as male and named Bruce at birth, having his penis accidentally destroyed during circumcision, and consequently being raised as a girl. He had genital reconstruction surgery performed on him to remove his testes, and was renamed as Brenda in an attempt to simultaneously give Dr. Money, the psychologist who oversaw his case a subject for an experiment concerning gender identity, research material, and give Bruce a chance at having a happy life as a female rather than live his life as a male with a mutilated penis. The fact that he bore the name David at the time of his suicide in 2004 shows that this went horribly wrong. Contrary to Dr. Money's claims that the reassignment was successful, David did not identify as female since his preteen years and began living as male when he was 15. The case was so famous that it was the subject of a documentary. In a reenactment of David's sessions with Dr. Money, they discuss the differences between males and females. At this time, David was still going by Brenda and still thought of himself as a girl.
Dr. Money: Brenda, how can you tell that I'm a boy and you're a girl? Brenda: Because I have long hair and you have short hair.