Northam performed at the Royal National Theatre - he replaced Daniel Day Lewis in the role of Hamlet (1989) and won the Olivier Award in 1990 for "most promising newcomer" for his performance in The Vosey Inheritance.
He made his American film debut in The Net (1995), but has appeared 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.
Presently, he can be seen portraying St. Thomas More on the Showtime series, The Tudors.
His audiobook work includes The Silver Chair by C. S. Lewis for Harper Audio and A Death Divided by Clare Francis for Macmillan