The son of two literary figures, he was educated at Kelly College at Tavistock in Devon and took a BA Hons degree in Modern History at University College London. Initially a journalist, he joined ITN in 1963, covering such topics as the Great Train Robbery, Vietnam, Ireland, the Munich Olympics massacre, Germany's Red Army, Italy's Red Brigades and Palestinian militant groups. His first book, Harry's Game, was published in 1975, and Seymour then became a full-time novelist, living in the West Country. In 1999, he featured in the Oscar-winning television film, One Day in September, which portrayed the Munich Olympics massacre.
Television adaptations have been made of his books Harry's Game, The Glory Boys, The Contract, Red Fox, Field Of Blood, A Line In The Sand and The Waiting Time.