Williams started out as a freelance journalist and creator of corporate videos. He decided to try comics writing resulting in Cla$$war, published by Com.x and illustrated by several artists, including Trevor Hairsine and Travel Foreman. It started published in 2002 and, after a three-year hiatus from the publisher a complete collection was released in 2009.
The comic series was read by Andy Diggle, who was the editor of 2000 AD at the time and contacted Williams, resulting in a number of series, first the two-part Asylum, with Boo Cook and then ongoing Low Life, initially with Henry Flint. and he would also produce one-offs there like Breathing Space with Laurence Campbell.
He would again team up with Campbell for the 2006 Wolverine Christmas Special and later a PunisherMAX one-shot. Williams would then go on to get other work for Marvel Comics, including a story in the Dark X-Men: The Beginning anthology, a Captain Britain story in Deadpool Team Up and a Ghost Rider one-shot that ties into the Daredevil storyline Shadowland.
Williams has also written comics based on a number of intellectual properties, including stories for Star Wars Tales and Rebellion comics from Dark Horse, and, with the same publisher, writing Indiana Jones and the Tomb of the Gods. For Dynamite Entertainment he is writing RoboCop, with artist Fabiano Neves.