Born Aug. 22, 1953, in southeastern London, O'Neill grew up in a working-class family. He started working in the comics industry at age 16, first as an office assistant for children's humor comic Buster. He eventually made his way to publisher 2000 AD, where he was both a writer and illustrator. Its stable of titles includes Judge Dredd and Nemesis the Warlock, which O'Neill created with Mills.
O'Neill eventually started getting work at DC Comics, beginning with The Omega Men, which is where he also began collaborating with Alan Moore (and put him in the sights of the Comics Code Authority, who threw a fit over his supposedly grotesque artwork). O'Neill and Moore would eventually co-create the infamous League of Extraordinary Gentlemen, along with its Nemo-related spinoffs. They also worked together on the lesser-known (but still quite good!) Cinema Purgatorio. In 2017, O'Neill even co-wrote a novel with author Pat Mills, about a serial killer in the comics industry that certainly isn't a metaphor for anything.
In Memoriam: Comic Artist Kevin O'Neill 1953-2022 [John Freeman / Down The Tubes]
How the late Kevin O'Neill changed comics for the better [Graeme McMillan / Popverse]