The Joker's exact origin is a mystery, barring that Batman was somehow involved. He revealed a possible story to Dr. Harleen Quinzel in Arkham Origins and to Professor Hugo Strange in Arkham City: once he had been a failed stand-up comedian who worked night shifts at Ace Chemicals in a feeble attempt to support his sickly wife Jeannie and their unborn child. They died just before the night that he helped a gang of thieves from The Falcone Crime Family break into the Ace Chemicals plant, disguised as the infamous Red Hood. Batman interrupted the heist, resulting in the would-be criminal falling into a vat of unidentified chemicals. Washed out of the plant he removed his disguise to find they had disfigured his entire body, giving him chalk white skin, green hair and his face a clown like appearance. Driven insane by this series of events he embraced it, believing that the greatest joke of all is that humanity is only one bad day away from total insanity.

Strange refuted this story however, claiming the Joker had probably fabricated it to avoid taking responsibility for whatever his actual actions had been and noting the clown had told at least ten different origin stories during his career. Joker admitted that even he didn't remember what happened and that the various stories were inspired by a friend who once told him: if you're gonna have an origin story, it may as well be multiple choice. It was entirely possible that this too was a lie.

Other versions of the story, told to Dr. Penelope Young, included being born in a small fishing village with thwarted aspirations of joining the circus and being the son of a police officer who was slain by the mob. The Red Hood persona, the Ace Chemical break in and Batman's presence seem to be the only consistent events in each retelling; the details of whether Batman threw the Red Hood into the vat, failed to prevent him from falling in or if the criminal jumped in to escape justice seemed to depend on Joker's mood at the time.

Even the Joker's own insanity may be called into question, as it never impeded his intelligence, ruthlessness and ambition or prevented him from mastering chemistry to produce his various toxins and gases. He may not have actually been insane, simply the purest kind of human evil. However, only a true madman could find humor in the tragedies the Joker did and he at least once suffered intense visual hallucinations, visualizing Batman as a huge, winged monster during his second defeat at the hero's hands at Blackgate.