Critchlow's comic book career began in the early 1980s, when he contributed to fanzines and informal publications. His professional career began in 1983 when his work was published in Issue 45 of Games Workshop's White Dwarf magazine, where Critchlow first portrayed his fantasy barbarian character, Thrud the Barbarian, in a regular, page-long, black and white, ink-drawn strip of the same name. Thrud was published for over fours years until issue 106; the strip was voted "most popular feature" for three years running in readers' polls. Thrud the Barbarian often reflected current Games Workshop product lines and borrowed themes from games like Judge Dredd, Blood Bowl and Warhammer 40,000 and Thrud's native fantasy theme. To celebrate the character's status as a popular feature of the publication, Citadel produced a number of metal miniatures of Thrud. Shannon Appelcline referred to Bil's Gobbledigook and Thrud the Barbarian as the two comics "for which White Dwarf is probably best known". Critchlow also provided numerous black and white interior illustrations for Games Workshop's Dark Future game, and was featured in an Illuminations exposè in White Dwarf issue 103.
In 1984, Critchlow had his debut in mainstream comic books when he contributed The Black Currant strip for Warrior issue 26. In the 1990s, after the Thrud strip had concluded in White Dwarf, Critchlow began working with 2000AD, and he contributed artwork for Pat Mills' Nemesis & Deadlock strip. Critchlow contributed art for numerous strips in 2000AD, including Tharg's Future Shocks, Judge Dredd, Mean Machine: Son of Mean Machine, Tales of Telguuth, Flesh, and full colour work for the 1995 crossover Judge Dredd/Batman: The Ultimate Riddle. In 1993, he also produced a piece of cover artwork for Aliens magazine, based on the graphic novella Aliens: Tribes.
Critchlow further contributed to the gaming world in 2000, and his work appeared in the Dungeons & Dragons third edition books Monster Manual, Monsters of Faerûn, Magic of Faerûn, Lords of Darkness, Tome of Magic and Fiendish Codex I: Hordes of the Abyss. He has also provided illustrations for the Wheel of Time Roleplaying Game and the Star Wars supplements Secrets of Tatooine, Ultimate Alien Anthology and Star Wars Hero's Guide and almost two hundred illustrations for the Magic: The Gathering card game. He also contributed to JLA: Riddle of the Beast, a DC Comics graphic novel by Alan Grant.
In 2000, Critchlow withdrew from mainstream comics; he wrote and drew the Thrud the Barbarian strip for its own comic. The series proved popular with the comic reading public and won the Diamond 2004 Award for Best Small Press title. During this time he also worked as a lecturer and numerous new comics artists, such as Barry Renshaw, credited him for helping and inspiring them. In October 2002, four months after the first issue of Thrud was published, Critchlow started drawing for 2000AD again. His first work was the Judge Dredd story Out of the Undercity written by John Wagner, followed by the introduction of new comic Lobster Random in 2003 with No Gain, No Pain, written by Simon Spurrier. This was followed by Tooth & Claw in 2004 and The Agony & the Ecstasy in 2006. His current work includes ongoing artwork for Lobster Random and artwork for Judge Dredd scripts by Gordon Rennie.