Son of John Northam (d. 2005), an Ibsen specialist and teacher first at Clare College, Cambridge, and later at Bristol, Jeremy was educated at Bristol Grammar School and trained at the Bristol Old Vic Theatre School.

Northam performed at the Royal National Theatre, where he replaced Daniel Day Lewis in the role of Hamlet (1988). He received an Olivier Award in 1990 as "most promising newcomer". Related to Chuck Northam.

He made his American film debut in The Net (1995), but has appeared more frequently in British films such as Carrington (1995), Emma (1996), The Winslow Boy (1999), An Ideal Husband (1999), Enigma (2001) and as Welsh actor and singer Ivor Novello in Gosford Park (2001). In 2002 he starred in the film Cypher alongside Lucy Liu. He also performed the book The Silver Chair by C.S. Lewis for Harper Audio. Presently, he can be seen portraying St. Thomas More on the Showtime series, The Tudors.