Firth was born in Grayshott, Hampshire, UK, 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 also an actor. Firth's parents were raised in India, because his maternal grandparents, Congregationalist ministers, and his paternal grandfather, an Anglican priest, performed missionary work abroad. Firth spent part of his childhood in Nigeria, where his father was teaching. He lived in St. Louis, Missouri when he was 11. He later attended the Montgomery of Alamein Secondary School, (now Kings' School), a state comprehensive school in Winchester, Hampshire, and then Barton Peveril College in Eastleigh, Hampshire. While in Kings' School, he wanted to play the guitar, but the school banned the guitar and saxophone, as they were "not serious instruments", and he was told to play the baritone euphonium instead. His acting training took place at the Drama Centre London.

In 1983, Firth starred as Guy Bennett 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 1986 he starred with Sir Laurence Olivier in Lost Empires, a TV adaptation of J. B. Priestley's novel, then in 1987 he appeared alongside Kenneth Branagh in the film version of J. L. Carr's, A Month in the Country. In 1989, he played the title role in the film Valmont, and was co-lead in the film Apartment Zero. Firth and other young British actors who were becoming established film actors such as Tim Roth, Gary Oldman, Bruce Payne and Paul McGann were dubbed the 'Brit Pack'.

It was through the 1995 BBC television adaptation of Jane Austen's Pride and Prejudice that Firth gained wider renown. The serial was a major international success, and Firth gained heartthrob status[citation needed] because of his role as Mr. Fitzwilliam Darcy. This performance also made him the object of affection for fictional journalist Bridget Jones (created by Helen Fielding), an interest which carried on into the two novels featuring the Jones character. In the second novel, Bridget Jones: The Edge of Reason, the character even meets Firth in Rome. As something of an in-joke, when the novels were adapted for the cinema, Firth was cast as Jones's love interest, Mark Darcy.[citation needed] Continuing this in-joke there was a dog called Mr Darcy in the film St. Trinian's which Firth's character accidentally kills.[citation needed]

Firth had a supporting role in The English Patient (1996) and since then has starred in films such as Fever Pitch (1997), Shakespeare in Love (1998), Relative Values (2000), Bridget Jones's Diary (2001), The Importance of Being Earnest (2002), Love Actually (2003), What a Girl Wants (2003), Hope Springs (2003), Girl with a Pearl Earring (2003), Bridget Jones: The Edge of Reason (2004), Nanny McPhee (2005), Then She Found Me (2007) with Helen Hunt, The Last Legion (2007) with Aishwarya Rai, When Did You Last See Your Father? (2008), the film adaptation of Mamma Mia! (2008), Easy Virtue which screened at the Rome Film Festival to excellent reviews and most recently he starred in A Christmas Carol (2009), the 50th adaptation of Charles Dickens's novel A Christmas Carol using the performance capture procedure, playing Scrooge's optimistic nephew Fred.

He has also appeared in several television productions, including Donovan Quick (an updated version of Don Quixote) (1999) and Conspiracy (2001), for which he received an Emmy nomination. Colin Firth's most recent role is in the Toronto International Film Festival debuted film, Genova.

At the 66th Venice International Film Festival in 2009, Colin Firth was awarded the Volpi Cup for Best Actor for his role in Tom Ford's A Single Man as a college professor grappling with solitude after his longtime partner dies. Fashion designer Tom Ford made his director's debut with this movie. This role has earned Firth career best reviews and Academy Award, Golden Globe, Screen Actors Guild, BAFTA, and BFCA nominations; he won the BAFTA Award for Best Actor in a Leading Role in February 2010.[citation needed]

Firth played William Shakespeare in a comedy special entitled Blackadder: Back & Forth. Edmund Blackadder runs into Firth's character while he is working on Macbeth, asks him to sign the script for him, and then punches him, saying "That is for every schoolboy and schoolgirl for the next 400 years!".

He was a guest host of Saturday Night Live in 2004 alongside musical guest Norah Jones.

Colin performed in theatre frequently between 1983 and 2000. He starred in Three Days of Rain as lead character Ned/Walker, as well as The Caretaker, Desire Under the Elms and Chatsky.

He served as executive producer for the 2007 documentary produced by his wife, Livia Giuggioli, In Prison My Whole Life. The film questions the trial proceedings and evidence used against political activist and former Black Panther member, Mumia Abu-Jamal, who is on death row for the 1981 killing of a Philadelphia police officer, Daniel Faulkner.