Marsan was born in Stepney, London to a working class family; his father was a lorry driver and his mother a school dinner lady and teacher's assistant. He was brought up in Bethnal Green and initially served an apprenticeship as a printer, before beginning his career in theatre, and moving to a television and film career. He trained at the Mountview Academy of Theatre Arts, graduating in 1991.

Marsan's first television appearance was in 1992, as a "yob", in the London Weekend Television series The Piglet Files. One of his more significant earliest television appearances was in the popular mid-1990s BBC sitcom Game On as a bungling bank robber. Marsan went on to have roles in Casualty, The Bill, Kavanagh QC, Grange Hill, Silent Witness, Ultimate Force, and more.

Marsan has since branched out into numerous and varied film roles including work in the United States, and is achieving much success in Hollywood, most recently as the main villain in the 2008 superhero film Hancock alongside Will Smith and as Inspector Lestrade in Guy Ritchie's Sherlock Holmes. His other movies include Gangs of New York, 21 Grams, The Illusionist, V for Vendetta, Gangster No. 1, Miami Vice, Mission: Impossible III, I Want Candy, Vera Drake, Happy-Go-Lucky and Heartless.