Parker's earliest work in comics was Solitaire, published by Malibu Comics. He later illustrated comic books published by DC Comics, Dark Horse Comics, and Image Comics, and worked as a storyboard artist on the television cartoon Big Guy and Rusty the Boy Robot.
His work as a writer at Marvel includes the limited series Agents of Atlas, First Class, and Marvel Adventures The Avengers.
Parker is also the writer of Walk-In and the second volume of Guy Ritchie's Gamekeeper for Virgin Comics.
Recent work includes "Fall of the Hulks." He will also be returning to Agents of Atlas with a new ongoing series to be called simply "ATLAS." He has also written a mini-series for Wildstorm, called Mysterius: The Unfathomable Parker also took over the writing of Thunderbolts with issue #138, introduced the Agents of Atlas in the following two issues and then took the title in "Siege," after which he will oversee an overhaul of the team line-up.