Firth was born in Grayshott, Hampshire, England, the son of Shirley Jean (née Rolles), a comparative religion lecturer, and David Norman Lewis Firth, a history lecturer and education officer for the Nigerian Government. Firth has a sister, Kate, and a younger brother, Jonathan, who is now also an actor. Firth's parents were born and raised in India, because his maternal grandparents, Congregationalist Read more...

In 1983, Firth starred as Guy Bennett in the award-winning London stage production of Another Country. In 1984, he made his film debut in the screen adaptation of the play, taking the role of Tommy Judd (opposite Rupert Everett as Bennett). In 1987, he appeared alongside Kenneth Branagh in the film version of J. L. Carr's novel, A Month in the Country. In 1989, he played the title role in the film Read more...

Firth's first published work, "The Department of Nothing", appeared in Speaking with the Angel (2000). This collection of short stories was edited by Nick Hornby and was published to benefit the TreeHouse Trust, in aid of autistic children. Firth had previously met Hornby during the filming of the original Fever Pitch.