He has also guest-starred in Van der Valk (1991), Inspector Morse (1992); Covington Cross (1992); Cadfael (1994); Agatha Christie's Poirot: Hickory Dickory Dock (1995); Tales from the Crypt (1996); Midsomer Murders (1997); Kangaroo Palace (1998), an Australian drama set in the 1960s; The Magical Legend of the Leprechauns (1999); The Inspector Lynley Mysteries (2002); Sparkling Cyanide (2003); and Jericho (2005). He appeared in The Prince and Me 2: The Royal Wedding (2006), The Prince and Me 3: A Royal Honeymoon (2008) and The Prince and Me 4: The Elephant Adventure (2010).

In addition to his roles in the theatre, Firth has acted in cinematic films and radio dramas, narrated audiobooks, and has also made notable television appearances, such as Linton Heathcliff in Emily Brontë's Wuthering Heights (1992); Fred Vincy in Middlemarch (1994); Sergeant Troy in Far from the Madding Crowd, for which he received a nomination for best actor; Lord Arthur Goring in An Ideal Husband (2000); and Prince Albert in Victoria & Albert (2001). He portrayed Joshua in the 2000 biblical film, In the Beginning.

After graduation, he toured with the Royal Shakespeare Company, where he made his debut at the age of twenty-seven as Henry VI (1994).

In 2003, he acted in the BBC's dramatised documentary Pompeii: The Last Day. That same year, Firth appeared in the film Luther, portraying Cardinal Aleander, the papal adviser who sought Luther's excommunication. For his performance, he received the CFT Excellence Award for Best Actor in a Motion Picture.

In 2008, Firth starred as the chef in a short film, The Chef's Letter, played Evan in the radio production of Dame Daphne du Maurier's September Tide, and was also featured in an episode of the American television series Ghost Whisperer. In 2009, he portrayed Dr. David Fuller in the German film production of Albert Schweitzer.

In 2016, he appears as Rex Bishop in the BBC series Father Brown episode 4.1 "The Mask of the Demon".