Screen and stage

Northam performed at the Royal National Theatre – he replaced both Ian Charleson and Daniel Day-Lewis in the role of Hamlet (1989) when they had to withdraw and won the Olivier Award in 1990 for "most promising newcomer" for his performance in The Voysey Inheritance.

He 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). He made his American film debut in The Net (1995).

In 2002, he starred in the film Cypher. That same year, he portrayed singer Dean Martin in the CBS film Martin and Lewis and golfer Walter Hagen in Bobby Jones: Stroke of Genius in 2004. In 2007 and 2008, he portrayed Thomas More on the Showtime series, The Tudors. He played John Brodie Innes in the 2009 film Creation, based on the life of Charles Darwin. In the 2015 film The Man Who Knew Infinity, he portrayed the philosopher Bertrand Russell. He played British Prime Minister Anthony Eden in the 2016 Netflix drama series The Crown.

Other work

His audiobook work includes The Silver Chair by C. S. Lewis for Harper Audio and A Death Divided by Clare Francis for Macmillan. For SilkSoundBooks, he recorded The Real Thing and Other Short Stories and The Aspern Papers, both written by Henry James. In 2007 he recorded Gerard Manley Hopkins poems for "The Great Poets" edition for Naxos Audiobooks. In 2009, he recorded Our Man in Havana by Graham Greene for CSA Word. He recorded the audio book Dark Matter, a ghost story by Michelle Paver, in September 2010; it was released on 21 October 2010, by Orion.

In the Gosford Park soundtrack, Northam sings the Ivor Novello songs "And Her Mother Came Too", "What a Duke Should Be", "Why Isn't It You", "I Can Give You the Starlight" and "The Land of Might Have Been" accompanied by his brother Christopher on piano.