Simpson started off in British comics, like Warrior and 2000 AD, and has since illustrated comic books for such industry heavyweights as DC Comics and Marvel Comics.
He has worked on such comics as Batman, Judge Dredd and John Constantine of Hellblazer.
Since 2000, Simpson has worked in the film industry, including storyboarding for City of Ember and Closing the Ring, and illustrating animated films. He is also partner in the Belfast-based animation company Rogue Rocket. Since 2011, Simpson has been producing storyboards for the television series Game of Thrones.