His earliest projects were programmed on the BBC Micro as a hobby and published when he was 17 years old. They were a graphical adventure Escape from Moonbase Alpha and a Donkey Kong clone Killer Gorilla (both published by Micro Power). Killer Gorilla became one of the best-sellers for the BBC Micro in the early 1980s and was ported to the Acorn Electron and Amstrad CPC. It also lead to Atarisoft commissioning Stephens to port Donky Kong Jr to the BBC. Stephens' other BBC Micro projects included an official port of Llamasoft's Colourspace, a clone of Mr. Do! for Micro Power called Mr. Ee! and a clone of Amidar for Superior Software called Crazy Painter.
During the late 1980s and 1990s, he worked on games for a wide range of platforms including Commodore 64, Atari ST and Amiga. One of his most successful games was an IBM PC simulation game Stunt Island, published by Disney Interactive in 1992.
In January 1997, Stephens set up Luxoflux with fellow games veteran Peter Morawiec. The team's first project was the well received Vigilante 8 for the PlayStation (later ported to the Nintendo 64 and Game Boy Color and given a sequel). Luxoflux was acquired by Activision in October 2002 and continued to develop games for multiple platforms including the popular True Crime series (see True Crime: Streets of LA and True Crime: New York City) and the official movie tie-in for Shrek 2.
In January 2007, Stephens, along with Morawiec, founded Isopod Labs which is a Research & Development studio which is currently working on a cross-platform development tool for next generation consoles as well as an as yet unannounced game (rumoured to be a remake of Vigilante 8).