Originally an engineer at the chemical company based in Gotham City known as Ace Chemicals, the unnamed man who became The Joker, felt like an outcast at his job due to being too much of a low-life, and quit to take up the career of a stand-up comedian, and eventually failed miserably. Desperate to earn money with his wife, Jeannie, heavily pregnant at home, the engineer turned to two criminals to help out, which they agreed to if he helped them through the chemical plant that he used to work at so that they could rob the Monarch Playing Cards company next door. To protect the engineer's identity during the robbery, the two criminals let the engineer take up the mantle of their gang's make-believe leader, The Red Hood, a criminal kingpin persona that was given to the inside man of every of the gang's jobs (yet was never the same man twice), which thus made the man appear to be the ringleader and allowed the gang to escape. During the planning, however, two policemen approached the engineer and informed him that Jeannie and his unborn child were killed in a household accident. After the police left, the engineer, distraught at his loss, suggested to the criminals to withdraw from the plan, only to be pressured to stick with it for them.
At the chemical plant, the robbery plot became doomed from start to finish. Since there was no security when the engineer worked there, the company must've altered it since he left, because when he entered it with the gang, the security guards spotted them and gave chase. Exchanging gunshots with them, the two criminals were gunned down and the engineer was cornered by Batman, who investigated the disturbance. Terrified of the "black demon" who was sent to punish him, the engineer begged to spare him, claimed that he was set up and not a crook, and even attempted to unmask himself. Unfortunately for him, he tripped over his cape, and fell into one of the chemical vats after the railing broke, despite Batman's efforts to save him. The engineer was able survive the trip through vat's drainage pipe and out into the river, although his skin, lips, and hair were permanently altered, bleaching his skin chalk-white, the lips ruby red and the hair emerald green. The effects of the process was enough for him to be driven insane and result with the birth of Batman's most encountering foe; The Joker, a humorist psychopath who quickly gained infamy within Gotham City for causing several criminal acts, including especially mass-murder, simply for his own twisted amusement, as well as crippling Dick Grayson, the first Robin, in the shoulder and Batgirl, forcing the latter to have retired back to her life as Barbara Gordon, in the hips, and leaving behind Joker playing cards as calling cards.
At some point, The Joker ended up hired by Ra's al Ghul to act as a distraction for Batman and Jason Todd, the second Robin, while the League of Assassins proceeded to topple the European Economy. To that end, the Clown Prince of Crime and several of his henchmen proceeded to raid a hospital in Bosnia with the seeming intention of raiding several medicine cabinets, and also poisoned several of the staff with Joker Venom only to be stumbled upon by Batman. However, in actuality, Joker was actually there to draw the Caped Crusader and Robin out. They then fought, with Joker laughing as his henchmen proceeded to attack them, although after Robin viciously knocked out one of his henchmen, Joker proceeded to make a run for it, and Robin recklessly pursued the psychopath. However, Joker turned the tables on Robin. Despite it being against Ra's intentions, Joker then abducted Robin, took him to a warehouse in the outskirts of Saravejo, and proceeded to sadistically torture him by beating him to an inch of his life with a crowbar. Eventually, Joker proceeded to leave the warehouse, and also rigged it with explosives with a short enough time limit to ensure that Batman could not rescue Robin in time, although not before he darkly told Robin that he tell "the big man" that he said "hello." Joker then left into the night, and the bombs blew up and killed Jason just as Batman arrived at the warehouse. Presumably after that, Batman managed to lock Joker up at Arkham Asylum, presumably increased security to such an extent that Joker could not hope to escape, and possibly put him into a body cast for six months.
Five years later, Joker was visited by Batman, and, after he cracked a joke about how the Caped Crusader needed to "get a lot of sun" despite his own pale complexion, was interrogated about any possible connection to the new Red Hood. Joker then mentioned that his outfit had horrible taste as his old uniform was flashy and more "maître d" than "motorcycle fetish". When Nightwing implied that they would find out if he was connected in any way to that new Red Hood, Joker proceeded to comment that Nightwing was not as fun since he was all grown up, before he cruelly alluded to his murder of Jason. That also nearly got Joker killed by the Dark Knight (although he inferred that he wanted Batman to kill him). Afterwards, Joker denied any involvement in the new Red Hood, and cited that if he was to do something like that, he'd make sure that Batman knew it was him, before he was left in his cell laughing hysterically. Unbeknownst even to the Joker, however, the new Red Hood WAS indeed connected to him, as he was in fact Jason Todd, who had been revived by Ra's and the League of Assassins via the Lazarus Pit due to guilt at his inadvertent role in his death at the hands of the clown-like sociopath. As such, Joker's release eventually was reluctantly engineered by the criminal kingpin, Black Mask, via various connections, including a lot of money and murder, after an attempt at his life by Red Hood in an attempt to fight fire with fire, with Joker then asked if it was an invite to a slumber party, and added that he packed his toothbrush. Joker then was brought before Black Mask while eating potato chips, where Black Mask told him that he was hiring him to take care of a problem, namely, murdering the Red Hood, as Joker had absolutely all the skills that were needed to take care of it. Joker then requested that he have a glass of water first. Joker then proceeded to use his glass of water to smash part of his glass, slit the throat of one of Black Mask's bodyguards, stole his hand gun and quickly killed the rest the body guards before they could get a chance to return fire. Joker then requested that he get his old outfit back, as well as a very big truck. After Black Mask accepted those demands and asked if he wanted anything else, Joker told him that he may need some guys, and added sadistically that he didn't want those guards since they're dead.
Afterwards, Joker then used a stolen transport truck to abduct the various drug lords under Red Hood's employ, then back stabbed Black Mask by forcing him and his assistant, Ms. Li into the trailer with the other drug kingpins, since they had formerly been under his employ. Joker then deliberately crashed the truck on the bridge, and caused enough of a spectacle for news choppers and the Gotham S.W.A.T. Team to arrive. Joker then proceeded to, while singing "I'm a Little Teapot, douse the various kingpins with gasoline, and addressed Black Mask's protests about how locking him and Ms. Li with the former drug lords was not part of the deal by citing their former affiliation with them while unveiling a lighter with the intention of torching them. However, the lighter was unable to operate, although Joker then noticed Red Hood's arrival, and repeated what he said earlier about his views about his outfit. Joker then told Red Hood that he had been in the middle of torching his gang, although Red Hood told him to go ahead, as he really didn't care if they died. Joker then admitted that he just wanted to get his attention as part of the job. Red Hood then revealed that he had always had his intention, and in fact, he actually wanted an audience with him. Joker then pointed out that this had implied that Red Hood was the one who orchestrated his jailbreak. Red Hood then confirmed it, and cited that while he had many plans, the endgame was to ensure that Black Mask got so desperate to take him down that he was willing to cut a deal to secure Joker's escape, since he was the only one with the connections to access Arkham Asylum and break him out of his relatively safe place. Joker then mentioned that he had been bamboozled, with Red Hood telling him not to undersell it, and told him that he had to make a lot of work for their reunion. Joker then noticed from his words that he's implying that they've met before, with Red Hood grimly confirming they had indeed met before. Joker then successfully lit the lighter, and dropped it into the truck after he mentioned "warm memories". However, Joker torching them was barely foiled by Batman via the Batwing's fire extinguisher foams, and then his attempt to retrieve Joker. However, Jason proceeded to cut the line and then made off with the Joker while they were underwater.
Afterward, Joker was dragged to an abandoned apartment complex near Crime Alley by the murderous vigilante. Joker then asked if they were going to do a slumber party, charades, or even a little truth or dare, before he requested dare. However, Red Hood then revealed a crowbar and ominously showed it to the Joker, which caused him to fearfully request truth instead as Red Hood slowly approached him before he proceeded to beat him with the crowbar, just as Joker himself did to him five years earlier. Joker then claimed that it would only hurt when he laughed. Joker then asked for Red Hood's identity, due to Red Hood's earlier comment about a reunion, and Joker beginning to recognize him from somewhere, asking if they doubled at the prom or blew up a school bus together. Red Hood then cryptically informed him that he actually knew him because Joker effectively helped create him, with Joker inquiring him to tell him more. Red Hood then "obliged" by grabbing him and then continued to beat him with the crowbar. Red Hood then proceeded to tie Joker up to a chair, locked him in a closet, and also rigged the fireplace with a cache of explosives as a fallback plan. Joker then was exposed by Red Hood, now unmasked as Jason, after Batman pursued him to the apartment complex, who demanded to know why Batman let Joker live, with Joker agreeing with Jason (by that point realizing the connection between Red Hood and Jason Todd), citing that he came back from the dead to have the "shindig" occur. Joker then proceeded to ask for a camera and take pictures of "[Joker] and [Red Hood]" first, then "[Batman] and [Joker]", then all three of them, and then one with the crowbar, although the last one had Red Hood angrily knock him to the floor and threatened him with shooting his groin first if he didn't keep as quiet as possible, with Joker calling him a "party pooper." Joker then ominously commented that Batman did think about him in a touched manner when Batman admitted that he did think of trying to subject him to every single instance of torture that he did unto others and then end his life. After Batman refused to violate his one rule, due to fears of becoming as much of a monster as Joker if he did, Red Hood then placed Joker in front of him, took out a gun, threw it to Batman, and demanded that he either let Jason kill the Joker, or shoot him right in the face (with Joker making no secret about how he was pleased at this scenario). Batman ultimately refused, and even threw a Batarang into Jason's gun barrel after the latter fired it while Batman was walking away to force him to choose. Joker then expressed amusement at that, and after congratulating Batman in "getting [Jason Todd]" and calling him an "expert, rooting-tooting, eagle-eyed, goth-loving marksman", expressed his joy at how despite Batman managing to find a way to win, everybody still loses. However, after noticing that Jason had activated the cache of explosives, Joker prevented Batman from disarming the explosives and told him not to spoil it as that situation was even better before he pinned him to the ground via strangulation while he gleefully declared that he would be the only one getting what he wanted that night, which was all three of them dying together before he laughed insanely at the impending countdown. However, Batman knocked Joker out and rushed to save Jason while Joker lain unconscious near the explosives. The explosives detonated, although Batman could only find a weakly laughing Joker underneath the rubble, with Batman angrily helping him out. Joker was then placed back in Arkham Asylum.
- Joker's prison uniform number was 11941, which was the issue and year that he made his first debut in comics.
- Although not directly confirmed, dialogue from the man who would become Joker in Batman's flashback to the day Joker was created implied that this version of the Joker shared the same backstory as the one depicted in Batman: The Killing Joke, or at the very least a similar backstory.
- That incarnation of the Joker was one of only two incarnations to have a very deep voice instead of his usual high-pitched voice, the only other being the incarnation played by Jack Nicholson in the 1989 Batman movie by Tim Burton.
- In addition, that incarnation was given a more stocky and somewhat imposing figure compared to his usual lean appearance. That was most likely due to the more darker aspect to the character and the film.
- That incarnation of the Joker was briefly referenced in the animated film: Batman: The Dark Knight Returns Part 2, where that film's version of the Joker killed Dr. Wolper on the David Endocrine show by smashing his novelty mug and used the shards to slit his throat, which mirrored a similar action conducted by the Joker in that movie when meeting with Black Mask (in the original comic, Joker instead had one of his robots snap the doctor's neck.).
- Similarly, that version's actions with the drug dealers were later repeated with the character Jerome Valeska and the Maniax in the TV series Gotham episode "Knock, Knock", where he doused an entire bus full of cheerleaders with fuel from a stolen truck that was meant to refuel ships at a dock and attempted to torch the bus and its inhabitants. Ironically, both also ultimately ended up foiled due, at least in part, to a temperamental lighter.
- Similarly, that version was also referenced in the final Joker origin flashback in the version used in Batman: The Killing Joke animated adaptation, where the man who became the Joker pleads to Batman to spare him only to end up tripping on his own cape and falling into the vat in a similar manner to the Joker in this movie (in the comic the film adapted from, the man instead jumped into the vat deliberately as a last ditch effort in trying to get away from Batman).