He was educated at Cardiff High School and University College, Cardiff, obtaining his M.A. in 1967. His best-known novel, Firefox (1977) became a successful Hollywood film. Other books include: Snow Falcon and A Different War. Although the invention of the techno-thriller genre is often attributed to the better-known Tom Clancy, many feel that Thomas was its true originator.

Most of Thomas's novels are set within MI-6 and feature the characters of Sir Kenneth Aubrey and Patrick Hyde.