Grim and all other supernatural beings were not real... At first.Mandy is so world-shatteringly powerful, that she can blur the line between metaphysical and physical. Luckily, Billy is so world-shatteringly stupid that the various Eldritch Abominations come out as the silly, quirky characters they are. If either Billy or Mandy were to die, the universe would dissolve into pure Darkness or Stupidity, as appropriate for the surviving one.
Grim is forced to repeatedly revive the characters whenever they die.Over the course of the show, many of the characters have been seen dying, only to return in the next episode(usually regarded as Negative Continuity). In one episode, Grim is seen as being capable of allowing someone to live longer by putting more sand into his or her lifetimer. Since Grim is forced to be Billy and Mandy's best friend forever, that could mean that in order to live up to his end of the bargain, Billy and Mandy literally have to live FOREVER, meaning that he must bring them back to life whenever they get themselves killed. When any other characters die, Mandy probably orders Grim to revive them as well so she doesn't lose any of her future minions. And when Grim dies...well, he's the Grim Reaper, so he can probably revive himself.
Mandy is the ultimate evil of the multiverse, although she's not entirely aware of it.This would of course, make her the Evil Counterpart of Haruhi Suzumiya.
Mandy's real parents are Sauron and Cruella de Ville.She was adopted at birth.
Mandy is the reincarnation of ChernabogRemember "Dream a Little Dream?"  That wasn't a nightmare. That was a past-life memory.
Mandy is Eris' daughterHer adoptive parents didn't really know how to pronounce and/or spell Dysnomia, and then, just named her Amanda... Or just Mandy, anyways. That's why Eris is so much time around them, even for being a deity. And Dysnomia was, indeed, smarter than her mom in the original mythology.
Mandy is Johan Liebert's daughterWell, it explains how she got so many Yottanazis in above troper's detector.
Mandy is Eris' and Johan Liebert's daughterWhat, it'd be awesome. Also, it would explain everything.
Grim, his granny, and Nergal are secretly high-class chefsWhy else would their food be so goddamn delicious?
The Series takes place in the early 80s, and Billy grows up to be Fry on Futurama.Simultaneously, Mandy went into business and marketing after overthrowing Santa Claus and taking over the toy shop. The jolly old man and his wife were replaced by evil robots and Mandy renamed herself "Mom" in order to execute her mass rebranding scheme. Fearful of her eternal fate, she cryogenically froze herself as an old lady and soon Mom's Robot Oil became feared the world over.
The Dune-esque future where Mandy has enslaved the world might have been averted......If only Grim hadn't revealed it to Mandy, thus diverting the flow of time into an unavoidable destination.
Sassy Cat actually stands for the Sassy Kat ShowclubThat's why Billy and Irwin love it so much.
Jack's Deal with Death was made to counteract his Deal with the Devil.
Grim stumbled across "monkey porn" in "The Secret Snake Club"When the Secret Snake Club used Grim to go to Shnissugah's website, before it finished buffering, Grim covers the screen and tells the SSC not to look while the audience hears monkeys screeching and hooting. The mind boggles, doesn't it?
Mandy is VickyIf we assume that the show takes place in the 90's and Fairly Odd Parents takes place in the 2000's, it would make Mandy the right age to be Vicky. Notice how in both shows her parents are scared silly of her, and the drawing styles are similar. She simply dyed her hair and took on a new name as to make people less suspious of her after her previous actions. As for the nose...It grew back. As for the personality, well...its obvious they're both Complete monsters. In one episode Mandy has a crush on a guy and starts being...normal. Vicky is also at her most normal when she is with teenage boys. Her parents had Tootie in the hope that a sister would balance her out. It didn't.
Mandy can tap into the dark forces of the Underworld.Due to being with Grim for so long,Mandy has absorbed the energy of the underworld.Due to her incredible willpower,she has been able to tap into that energy.She used this to overwhelm Grim's magic and control the inhabitants of Endsville.This has also caused her personality to alter from a Jerkass to the Anti Christ
Billy is Obfuscating Stupidity.The truth is Billy is the most intelligent person in the universe. He simply has to contain it as it would cause reality itself to distort around him
In "My Fair Mandy", reality was broken by Mandy forcing a smile.Mandy has been seen to smile other times in the series, without the same consequence. Notably her true, non-evil smile in the episode "Crushed", where she falls in love. Reality fell apart not because she smiled, but because she forced a smile. (Also, in the Christmas episode, Grim and Count von Goulash held her mouth open in a smile, so perhaps that didn't count because she wasn't forcing it?)
Nergal is Slenderman.My first thought when I first heard about Slenderman. Nergal is a lanky humanoid creature in a suit, with retractable tentacles who kidnaps children. Can't be a coincidence.
Grim will eventually kill off Billy and MandyThink about it - sure, the deal was that Grim would be Billy and Mandy's "friend" forever, but he never promised them immortality, right? At least as far as I know anyway. I think he's just faking being stuck with them, and there's probably some massive backstory concerning Mandy being such a Karma Houdini. One day, Grim will kill Billy, and Mandy will try to avoid him, but he'll catch up eventually, and then turn the tables on her. He might even throw in an Ironic Hell, putting her in a similar situation to that episode with the mask (forget what it was though). *
Mandy is the Anti-Christ!It is really obvious only something as evil and wickedly wicked could be the father of Mandy. The Devil had naughty-love with Mandy's Mom, possibly disguised as Mandy's supposed dad, and impregnated her with Mandy. She already became president of Earth during the Underfist special, it's only a matter of time she gets to (or above) her father's level!
Mandy is Rose Lalonde.From Homestuck. Both are incredibly smart, blonde, and have a love of all things evil. Their associated colours are both pink and they have very similar haircuts. The events of "The Prank Call of Cthulu" inspire her to research Eldritch Abominations. Her mom hid behind a veil of Obfuscating Stupidity and changed Mandy's name to Rose when they left Endsville. Living alone, she learned how to like people (and thus became friends with John, Dave and Jade).
"Keeper of the Reaper" was one big analogy for/Take That at the American Supreme Court.
Mandy is the daughter of Him and ErisIt was a dare by the Red Guy. When Eris realized she was having a child, she switched the baby with a random one at the hospital, which happened to be Claire and Phillip's. And since Mandy is the daughter of the goddess of discord and the lord of the underworld, monsters and super natural beings are attracked to her like Billy to pie. Plus, the wolves actually wanted to kill her, they lied.
Billy and Mandy is a prequel to Adventure TimeAccording to the Adventure Time episode "Son of Mars", Abraham Lincoln used to be immortal, but gave up his immortality and traveled back in time to be killed by John Wilkes Booth. In Billy and Mandy, he's inexplicably still alive and appears to still be President. Clearly, Billy and Mandy is set in the timeline before Lincoln allowed himself to be killed in the past. The many supernatural forces in the show were partially responsible for how post-Mushroom War Earth became so bizarre.