The short answer
The Joker has no single origin story. DC has published several, contradicted them, and left the character without a firm past on purpose. The version most readers and filmmakers come back to is from Batman: The Killing Joke (1988) by Alan Moore and Brian Bolland, which follows a failed stand-up comedian who turns to crime, loses everything in one bad day, and falls into a vat of chemicals. That story is framed as a memory the Joker himself may not trust.
The Killing Joke origin
In that graphic novel, the man is a struggling comedian with a pregnant wife and no money. He agrees to help two criminals rob a chemical plant, taking them through the Ace Chemicals building as a shortcut. Before the job, he learns his wife has died in a household accident. The robbery goes wrong, Batman arrives, and the comedian jumps into a vat of chemical waste to escape. He surfaces as the Joker: white skin, green hair, a smile frozen onto his face.
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Moore keeps the whole thing unreliable. The Joker says he remembers it one way, then another. He is telling the story to prove that anyone can be broken by one terrible day.
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The earlier versions
- Batman #1 (1940): The Joker simply appears. He has no name, no family, and no explanation. He is a killer with a gimmick.
- The Man Behind the Red Hood (1951): Detective Comics #168 shows a criminal named Red Hood who falls into a chemical tank while fleeing Batman. The disfigurement is the birth of the Joker, and this is the first time his face is explained at all.
- Bronze Age and after: Writers added small details and then walked them back. DC has never made any of them fully binding.
Film and television takes
Tim Burton's Batman (1989) gave him a name, Jack Napier, and made him the mugger who killed Bruce Wayne's parents. That version locks the two characters into one event.
The Dark Knight (2008) does the opposite. The Joker tells a different story about how he got his scars each time he tells it: a drunken father, a gambling wife. He never repeats himself.
Todd Phillips' Joker (2019) builds a full origin for Arthur Fleck, a failed clown and stand-up comic in Gotham. It borrows the chemical-failure shape of the Killing Joke story while making the character sympathetic in a way the comics rarely allow.
The Gotham TV series went a different route again, with Jeremiah Valeska. Nearly every adaptation invents its own answer.
Why DC keeps it vague
An origin is a leash. Give the Joker a fixed childhood, a wife, and a motive, and he becomes explainable, and the character only works when he is not. The Killing Joke works precisely because the audience cannot tell whether the comedian's story is a confession or a performance.
What the versions do agree on is thin but consistent: a man with an ordinary life, one catastrophic day, and a chemical accident. Everything else is negotiable.