Jack Napier was a Hero who became The Joker. He started out as a chauffeur and hitman for the mob triad of Salvatore Valestra, Buzz Bronski and Chuckie Sol. Twice on separate occasions, Jack saw Bruce Wayne and exchanged mean glances at one another years before they would clash as The Joker and The Batman. One of his last jobs took place in Europe on the Mediterranean Coast. His target was businessman Carl Beaumont who he had embezzled from them. His daughter and Bruce's former fiancée, Andrea, returned home from grocery shopping only to see Napier emerging from the house. He silently walked past her grinning as she rushed inside to find her father murdered in cold blood.
Years later after the Valestra Mob had gone their separate ways, Napier struck out on his own and formed a small gang. The night they broke into the ACE Chemical Plant would mark the first time the pre-Joker would encounter The Batman. During their fight, Jack fell off a catwalk and into a drainage vat of chemicals. Since this event was never seen, though it was once dramatized for a news program, no one knows if Jack simply fell or was pushed by Batman as the Joker adamantly maintains. Regardless, what followed changed his life forever. He was washed out into the bay and discovered the chemicals bleached his skin chalk white, dyed his hair green, stained his lips red which left them in a permanent rictus smile and dyed his clothing purple. All this snapped his already unstable, sadistic mind and drove him to eternal insanity. He reinvented himself as The Joker, Batman's greatest enemy and Gotham City's most dangerous costumed criminal.
*In Batman: Mask of the Phantasm (1993), Joker's former identity was not referred to by name. His real name was previously established in the episodes "Dreams In Darkness" when it is spoken to Batman by Dr. Bartholomew and "Joker's Wild" where it is written in a dossier. It should be noted that the writers of the film were cautious of having Joker in the story since his origin was very close to what was seen in Batman (1989) and thus might draw attention away from the overall plot. This may be why Jack only appears briefly and has no dialogue.
**The use of this origin was due to the first three seasons being heavily patterned after the success and influential tone of Tim Burton's movies. However when the revamped fourth season began, during the era of Joel Schumacher's movies, Joker's origin was retained but his identity was retconned as being merely one of many aliases as seen in the episode "Beware the Creeper." This reflects the efforts of the writers to put the character back in line with his conflicting multiple origins in the comics.