To settle all questions, contrary to popular belief (in ebay listings) The Batman Adventures: Mad Love #1 is NOT the 1st comic appearance of Harley Quinn.
The Batman Adventures: #12 appears by all research available to be the 1st comic book appearance of Harley Quinn. Batgirl: Day One has a plot where Babs Gordon goes to a costume party dressed as Batgirl and winds up foiling a kidnapping by Quinn and Ivy and a diamond heist by Catwoman.!
Date: 1-Sep-1993 Writer: Kelley Puckett
The Batman Adventures: Mad Love (Special) Date: 2/1/1994 is the 1st TOLD ORIGIN OF HARLEY QUINN in comic books!
The Batman Adventures: Mad Love #1 was reprinted (2nd & 3rd printings) with a different cover:
1st Print Cover (left): 2nd & 3rd Printings
The Batman Adventures Annual #1 Date: fall 1994 is the 3rd comic appearance Harley Quinn (no other Batman Adventures comics had featured her since the February 1994 1st edition of Mad Love)
Batman: Harley Quinn #1 or NN; Dated 1/1/1999–is also “not” the 1st appearance, but is a more detailed Origin of Harley Quinn; Prestige format
Written by Paul Dini, with a more detailed origin, this novel brought Harley Quinn “into the DC Universe Mainstream” instead of just being an “animated character” Think of it as though the Wonder Twins in Super-friends were suddenly having major DC writing & a mini-series to launch their own publication!
a more affordable version came out in 2014!
ORIGIN OF HARLEY QUINN:
The Joker plays a winning hand at Arkham Asylum when he tricks his new psychologist, Harleen (“Harley”) Quinzel, into feeling sympathetic for him. Quinzel, a would–be pop psychologist with a flawed past of her own, falls madly in love with the Clown Prince of Crime. After the object of her affections escapes the asylum and Batman returns him—badly scathed—she springs him herself, having donned a clown costume herself and calling herself Harley Quinn (“Harlequin“).
In “Love on the Lam-Harley & Ivy” Poison Ivy helps Harley Quinn and their relationship in the DCUniverse strengthened.
Eventually, the Joker grew tired of her and attempted to blast her out of his existence. Harley is rescued by Poison Ivy (see the 1999 Novel above), whose antitoxins increase her natural agility and strength. And she has a score to settle with her old boss…