McMahon started being paid to draw comics in 1976, when he was asked by Pat Mills to draw Judge Dredd for 2000AD. For 2000AD he also worked on RoBusters, ABC Warriors, and Slaine.
Since then he has drawn for various publishers such characters as Batman, The Last American, Sonic the Hedgehog, Hellboy, Alien Legion, and Tank Girl. During the latter part of his career (if what he does can be described as a career) his comics work has taken a back seat to doing concept art and storyboards for movies and games.
As of this writing he is semi retired, mainly working on private commissions and The Kestrels, a series of graphic novels written by Ben Dickson.