Jason Flemyng was born in Putney, London, the son of Scottish television and film director Gordon Flemyng. He decided he wanted to become an actor after appearing in theatrical productions at his school, Christ's Hospital in Sussex. In the 1980s, he was involved with the National Youth Theatre and the political organisation the Young Socialists. He also became involved with Militant, an entryist group active within the Labour Party, and in 1987 was expelled from Labour for selling the Militant newspaper.
Flemyng has subsequently claimed that both his theatrical and political activities at this time were simply a way of meeting girls. In 1990 he was admitted to the London Academy of Music and Dramatic Art (LAMDA), where he was a classmate of Hermione Norris. Following his graduation from LAMDA in the early 1990s he joined the Royal Shakespeare Company.
- Lock, Stock and Two Smoking Barrels as Tom
- The Curious Case of Benjamin Button as Thomas Button
- Snatch as Darren
- X-Men: First Class as Azazel
- Clash of the Titans as Calibos / Acrisius
- Rob Roy as Gregor
- From Hell as Netley, the Coachman
- The League of Extraordinary Gentlemen as Dr. Henry Jekyll / Edward Hyde
- Alive and Kicking as Tonio
- Doctor Finlay as Dr. David Neil