Adlard began his work in the UK on White Death with Robbie Morrison and 2000 AD series including Judge Dredd and Armitage. He has continued working for them with the series Savage.
In the United States he is best known for his work on The X-Files (for Topps), Astronauts in Trouble (for AiT/Planet Lar) and The Walking Dead (for Image Comics). He has been the primary artist on The Walking Dead since 2004.
Other work includes: Mars Attacks! (also for Topps); The Hellfire Club and Warlock for Marvel Comics; Batman: Gotham Knights and Green Lantern/Green Arrow for DC Comics; Aliens magazine for Dark Horse Comics, White Death for Les Cartoonistes Dangereux; Blair Witch: Dark Testaments and Codeflesh for Image; Shadowman for Acclaim Comics; and The Establishment for Wildstorm, amongst many others.
He illustrated a graphic novel, Playing the Game, written by Nobel Prize winner Doris Lessing.
In 1992, he collaborated with best-selling horror writer Guy N. Smith on Crabs' Fury. The one-shot was re-released as a limited edition in September 2008 through Ghostwriter Publications.
In 2012, Adlard was one of several artists to illustrate a variant cover for Robert Kirkman's The Walking Dead #100, which was released July 11 at the San Diego Comic-Con.