He appeared in a number of films made by The Comic Strip Presents... (notably The Bullshitters, a parody of The Professionals) on Channel 4 in the early 1980s after becoming one of the breakthrough acts of the Comedy Store in 1979. The brother of comedian and film director Kevin Allen, Keith Allen has done both straight and comedy acting, once playing Brian Dennehy.

During the brief period of British Satellite Broadcasting as an alternative satellite broadcaster to Sky, he had a regular comedy show of his own I Love Keith Allen on the Galaxy channel, a mix of stand-up and sketches.

He appeared briefly in the black comedy, Twin Town (directed by his brother Kevin), the Channel 4 adaptation of A Very British Coup and played the lodger who dies at the beginning of Danny Boyle's thriller Shallow Grave (1994). In the same year, he turned in a critically acclaimed performance in a BBC adaptation of Martin Chuzzlewit. He was used again by Boyle to play a drug dealer in Trainspotting (1996). He also appeared disguised as a fictional hip-hop star 'Keithski' to present Top of the Pops on 19 July 1996.

In 2000, Allen appeared in two Harold Pinter plays at the Almeida Theatre, playing Lambert in Celebration and Mr Sands in The Room. These were performed again at The Lincoln Center Festival in July, 2001.

In 2001, he played the dark character of "problem-solver" Jim Napeworth in an episode of Murder in Mind, and in 2004 cameod in Black Books as poker-player Dave 'Mouse Ears' Smith.

He also appeared in the hard-hitting hospital drama, Bodies, as Mr Tony Whitman, a sarcastic but somehow likable Consultant Obstetrician with an enormous ego. In 2005 he appeared in the Endemol-produced BBC Two television programme Art School alongside Ulrika Jonsson, John Humphrys and Clarissa Dickson Wright where he discovered a passion for painting. From 2006 to 2009, Allen co-starred in the BBC's Robin Hood drama series, as the Sheriff of Nottingham.

Allen presented the TV show Whatever You Want in 1982, during the early days of Channel Four and has presented a number of TV documentaries for Victor Lewis-Smith's Associated-Rediffusion Television Productions: Little Lady Fauntleroy (2004), You're Fayed (2005) and on Michael Carroll - King of Chavs (2006). In 2007 his documentary Tourette De France appeared on Channel 4, in which he travelled with a group of Scottish people with Tourette syndrome, most notably John Davidson, on a Routemaster bus from London to the Parisian hospital where this condition was described by Georges Gilles de la Tourette in 1884. Keith Allen Will Burn In Hell appeared on Channel 4 in June 2007, and showed Allen profiling the controversial Westboro Baptist Church, led by Fred Phelps, and speaking to various members of the church and Phelps' family.

Allen presented the Manchester Passion, a contemporary retelling of the last few hours in the life of Jesus on Good Friday, 14 April 2006.

Allen's most recent work to date was "Keith Meets Keith", screened on 14 September 2009 on Channel 4, in which Allen tracked down TV chef Keith Floyd. The show contained what turned out to be Floyd's final interview for television; Floyd died of a heart attack on the evening the documentary was screened.

He was a member of Fat Les, a band which also contained artist Damien Hirst and Blur bassist Alex James. Allen was also closely associated with the band New Order. He co-wrote their only UK number one single, "World In Motion", and occasionally performed with them live, such as when New Order headlined the Reading Festival in 1998. He also appeared in the band's DVD New Order Story, where he played the host to a fictional New Order game show. He has been involved in several other football-related records, including "England's Irie" by Black Grape and wrote the lyrics for "Vindaloo" by Fat Les. He also contributed the song "On Me Head, Son" to the film England Manager, credited on the soundtrack album to Sporting Les. He also once danced onstage with the Happy Mondays in New York City.

Although born in Swansea, Allen is a fan of Fulham F.C., the oldest professional Football club in London. He has produced a number of official songs for the club with his band Colin and the Cottagers singing with the club's chairman, and former owner of Harrods, Mohamed Al Fayed. These include "We're Not Real Madrid" and "Back Home", a reference to the club's return in 2004 to its historic home Craven Cottage on the banks of the River Thames.

Allen was born in Gorseinon, in Swansea, Wales, the son of Edward Charles Owen, a petty officer in the Royal Navy, and elder brother of Kevin Allen. Allen lived for most of his childhood in Gosport (While attending Brune Park Community College) while his father served in Portsmouth. As a guest on Top Gear on 9 December 2007, Allen said claims he had 8 children were not true and that he actually has six children by four different women. His children include pop singer Lily Allen and actor Alfie Owen-Allen with his ex-wife Alison Owen. He was also married to Nira Park and had an affair with Julia Sawalha. He now lives with actress Tamzin Malleson, who starred alongside him in Bodies. Their daughter, Teddie, was born in 2006.