Sargent started playing Dungeons & Dragons in 1978 through friends. TSR UK were based in Cambridge, and they met with Sargent after he had submitted an article to Imagine magazine. The TSR UK crew later left to work for Games Workshop.

Sargent authored various Fighting Fantasy gamebooks and novels for Games Workshop from 1988-1995, some under the pseudonym Keith Martin.[3]:46 In 1989 Games Workshop spun off its sole remaining RPG line, Warhammer Fantasy Roleplay, into a new subsidiary called Flame Publications; Sargent was one of the freelancers that aided this new company.[3]:50 Sargent still did work for TSR, and his From the Ashes (1992) supplement pushed the Greyhawk world into a more conflictive period.[3]:25

He later worked as a freelance designer, and was brought in by TSR to work on Greyhawk. Most of his role-playing works were published between 1987 and 1996. He has authored many products for the Dungeons & Dragons (particularly for the World of Greyhawk setting), Warhammer Fantasy Roleplay and Shadowrun roleplaying games.