No other character in comic book history has had quite the complex relationship between comics and television as Harley Quinn. She first appeared in the 1992 episode of "Batman: The Animated Series," "Joker's Favor." The episode originally involved a scene where the Joker would burst out of a giant cake at a celebration for Commissioner Gordon in order to attack the event. The showrunners for the series, Paul Dini and Bruce Timm, thought that might look odd so they decided to invent a female henchman for the Joker who could do the cake gag instead.
A few months after the comic book debuted, Harley's past as a doctor was referenced on the TV series. In 1997, three years after the original "Batman: The Animated Series" aired its last new episodes, the show was moved from Fox to the WB, to be paired with the new "Superman: The Animated Series." WB wanted new episodes of the series to promote the pairing, so "The New Batman Adventures" began with 24 all-new episodes that extended the original Batman series. Dini and Timm took this opportunity to pull off something rarely accomplished -- they adapted a holiday comic book story starring Harley and Poison Ivy as well as "Mad Love." They wrote two comic book stories based on the animated series and then animated the comic stories based on the animated series. Things with Harley had come full circle!
When the Joker rebuffs her and tries to kill her, she barely survives thanks to Poison Ivy's help (who she meets for the first time in the DC Universe). Ivy uses chemicals to give Harley superpower, bestowing upon her superhuman acrobatic and fighting skills, which she shows off against the Joker and Batman. While she intends to get her revenge against the Joker, she is eventually wooed over to his side once again. Harley accompanies the Joker throughout the rest of the "No Man's Land" crossover, which ended in December of 1999.
Harley was again stood by the Joker's side during the summer 2000 Superman crossover "Emperor Joker," in which the Joker gained the powers of Mr. Mxyzptlk. At one point in the story, he actually transformed Harley into a constellation!
Soon after "Emperor Joker" wrapped up (with everything returned back to normal, of course), Harley received her own ongoing series written by Karl Kesel and drawn by Terry and Rachel Dodson. In the series, Harley went solo and eventually formed her own gang. She also moved from Gotham City to Metropolis. Kesel's final storyline involved Harley Quinn actually dying and going to hell. She eventually escaped and spent several issues attempting to regain corporeal form. Kesel's run finished with "Harley Quinn" #25. The new creative team of AJ Lieberman, Mike Huddleston and Troy Nixey took over with #26 and brought a much darker edge to the series for the final issues of the book, which ended with #38. At the end of the series, Harley, realized she needed help and checked herself in to Arkham Asylum.
Harley Quinn was out of the picture for the next couple of years, which by today's standards seems hard to imagine, but between 2004 and 2006 she remained primarily in Arkham Asylum (save for some comics set in the animated universe)
Paul Dini brought her back into the mainstream DCU mix with 2007's "Detective Comics" #831, where readers learned she had truly reformed. Dini then worked her into the year-long series "Countdown to Final Crisis," where a reformed Harley gave up her clown persona and started working at a women's shelter run by the Amazons (Harley was even dressing like an Amazon). She befriended Holly Robinson, the short-lived replacement for Selina Kyle as Catwoman, and they became involved in an adventure with Mary Marvel. The trio fought Granny Goodness to help rescue the Olympian gods and Harley temporarily gained new powers but lost them by the end of the series.
In 2009, Dini next returned Harley back to her clown persona in the ongoing series "Gotham City Sirens," where she, Poison Ivy and Catwoman all lived and worked together as reformed heroes. This arrangement only lasted a couple of years before both Harley and Ivy eventually slid back into villainy. By the end of the series, Harley reunited with the Joker and commited a series of violent crimes. The team broke up with Catwoman granting them one last kindness by helping Harley and Ivy to avoid capture by Batman.
This led into the New 52, where Harley received a dramatic makeover, wearing much less clothing but also having bleached skin and red and black hair. Her New 52 iteration is also far more violent and unhinged. Her new origin is roughly the same as her original "Mad Love" one, with the noted exception that once she hooks up with the Joker, he pushes her into a vat of chemicals. She survived the experience, but that explained her bleached skin. She has become a regular member of the Suicide Squad, serving with the team throughout the first volume of the New 52 "Suicide Squad" and continuing with the group in their current series, "New Suicide Squad."
Recently, actress Margot Robbie was cast as Harley Quinn in the upcoming film adaptation of "Suicide Squad" (with Jared Leto as the Joker). For a character created in television and molded in comics, film seems to be the logical next step!