Harley’s origin story was actually first told outside of the main DC Universe, in THE BATMAN ADVENTURES: MAD LOVE (1994). Taking place within the same continuity as Batman: The Animated Series (where Harley Quinn initially debuted in the 1992 episode “Joker’s Favor”), the story established Harley’s origin as an Arkham Asylum psychiatrist meeting The Joker for the first time, falling in love with him and eventually succumbing to her own insanity to become his sidekick and girlfriend.
Harley’s first official appearance in the main continuity of the DCU happened in BATMAN: HARLEY QUINN #1 (1999) as part of the year long “No Man’s Land” crossover event.
Following the events of 1998’s “Cataclysm,” Gotham City had collapsed into an earthquake-ravaged wasteland run by warring factions, including the Batman Family, the GCPD and gangs of various criminals and Super-Villains. BATMAN: HARLEY QUINN #1 adapted Harley’s MAD LOVE origin story, explaining that she was able to break out of Arkham, where she had been an inmate since her own psychotic break as The Joker’s psychologist, thanks to the chaos caused by the Cataclysm earthquake.
In this version of the story, The Joker launched Harley on a rocket in an attempt at murdering her, before confronting the reality of their relationship. The rocket, however, only managed to launch Harley into Robinson Park, an area of the No Man’s Land that Poison Ivy had taken over. Ivy found Harley and injected her with a toxin that rendered her immune to Ivy’s pheromones and other venoms, as well as granting her enhanced strength and durability—enough to allow her to hold her own in hand-to-hand combat against Batman himself.
From this point, Harley became a fixture of Gotham City, both as a sidekick to The Joker and an independent Super-Villain in her own right.
From Secret Six to Countdown (2007-2009)
Following a brief stint in Arkham Asylum and willing to try anything once, Harley appeared to fill an opening in the Secret Six in BIRDS OF PREY #105. She quit after her first mission.
Eager to put her life with The Joker behind her, Harley left Gotham in COUNTDOWN and set up shop at a women’s shelter run by Amazons in Metropolis, where she made friends with fellow Gotham expat Holly Robinson. The shelter took Harley to Themyscira, where both she and Holly were made to train as Amazonian warriors. Unfortunately, it was revealed that the Amazon they’d believed to be Queen Hippolyta was actually a disguised Granny Goodness, who used the shelter as a front to train a new army of Female Furies for Apokolips. Harley was then forced to fight through the fires of Apokolips before returning to Gotham City.
Following her return to Gotham, Harley formed a team with Poison Ivy and Catwoman called the Gotham City Sirens. Though not completely a force for good, the Sirens largely spent their time working to keep Gotham safer—even if it meant focusing on personal vendettas and crossing lines Batman and his friends never would.
Things began to fall apart, however, when Harley broke into Arkham with the goal of killing The Joker once and for all. Instead, when she reached his cell, he convinced her to rejoin him and betray Ivy and Selina, fracturing the Sirens’ bonds and setting off a cascade of backstabbing among the three women.
The New 52 (2011-2016)
Following FLASHPOINT (2011), Harley was given a new origin story that involved The Joker forcibly pushing her into a vat at Ace Chemicals in order to transform her physically the way he had been transformed. The traumatic event splintered their relationship, and left Harley dangerously unstable and less anchored in Gotham than she’d been in the past.
In SUICIDE SQUAD #1 (2011), Quinn was drafted into the Suicide Squad by Amanda Waller after being captured by Black Canary. She reluctantly served with the team, though her obsession with The Joker kept her preoccupied.
Eventually, upon learning The Joker had been killed (in truth, he’d merely gone into hiding), Harley's mental state crumbled. She wasted no time in betraying the Squad by SUICIDE SQUAD #5 (2012), and escaped to Gotham to try and find evidence of The Joker’s demise. Later, when The Joker was revealed to be alive in DEATH OF THE FAMILY, Harley realized she was no longer in love with him and opted to remove herself from his life completely.
Following her Joker epiphany, Harley left Gotham and moved to New York’s Coney Island with the goal of starting a new life away from the baggage of her past. In her first-ever solo ongoing series, Harley evolved yet again, gaining a brighter, more positive personality and the uncanny ability to break the fourth wall.
In this incarnation, Harley was a true antihero focused on doing her part to better her neighborhood, even if her methods were sometimes questionable. She began making more connections in the larger DC Universe outside of the Batman Family and The Joker’s network of rogues. She became friends with Super Heroes like Power Girl, and even went on to gather her very own “Gang of Harleys”—a troupe of mercenaries inspired by her aesthetic. She teamed up with various heroes in HARLEY’S LITTLE BLACK BOOK (2016), and toured the DC Universe unhindered, “befriending” (or bothering) Superman, Green Lantern and Wonder Woman, among others.
During this time, she became a landlord for her apartment building, the owner of a traveling sideshow and a member of a Roller Derby team, and all while remaining a member of the Suicide Squad.
In the limited series POISON IVY: CYCLE OF LIFE AND DEATH (2015), Harley and Ivy officially repaired their fractured relationship.
Harley’s status quo was largely unchanged after the events of DC UNIVERSE REBIRTH #1, continuing her adventures on Coney Island in HARLEY QUINN (2016) while serving hard time at Belle Reve Penitentiary in SUICIDE SQUAD (2016). How she manages to maintain her independent lifestyle while functioning as a full-time member of the Suicide Squad, however, is anybody’s guess. Harley Quinn’s never had any interest in making sense, after all. Why start now?