Sewell was born in Twickenham in the London Borough of Richmond upon Thames in South West London, the son of William, an Australian animator, and Jo Sewell, a Welsh artist and waitress. His father worked on the "Lucy in the Sky with Diamonds" segment of animation for The Beatles' Yellow Submarine film. His parents divorced when Sewell was five, and his mother worked to support her two sons. His father died when Sewell was 10. Sewell has said that he was a difficult teenager.
Sewell was educated at Orleans Park School, a state comprehensive school in Twickenham, which he left in 1984, followed by West Thames College, where a drama teacher sent him to audition for drama school. He later enrolled at the Central School of Speech and Drama in London.
After graduating, Sewell was set up with an agent by Judi Dench, who had directed him in a play while at the Central School of Speech and Drama. His breakthrough year was in 1993, in which he starred as the nasty Tim in Michael Winner's film Dirty Weekend. Winner chose him after seeing him in a play at the Criterion Theatre. Also in 1993 Sewell starred in the BBC serial of George Eliot's Middlemarch and on stage in Tom Stoppard's play Arcadia at The Royal National Theatre (Lyttelton). His film work includes 1995's Cold Comfort Farm, directed by John Schlesinger, the lead role of John Murdoch in the science fiction film Dark City in 1998, Amazing Grace, The Illusionist and Nancy Meyers' romantic comedy The Holiday. Amazing Grace deals with William Wilberforce's political fight to abolish slavery in Britain, with Sewell playing Wilberforce's co-campaigner Thomas Clarkson. Sewell is known for his villainous roles, such as those in A Knight's Tale, The Legend of Zorro, Bless the Child, Helen of Troy and The Illusionist. He spoke of his unhappiness about this, saying that "[I] don't want to play a baddie again." "Everyone has their thing they have to get around", notes Sewell. "With me, it's like okay, how can I make this upper class bad guy in the 19th century different and interesting?"
In 2008, Sewell appeared in the HBO miniseries John Adams as Alexander Hamilton. He received critical praise for his portrayal of "merry monarch" Charles II in the BBC's Charles II: The Power and The Passion. The series' cast included Ian McDiarmid, Helen McCrory, Rupert Graves and Shirley Henderson and spanned the life of the king from his last days in exile to his death. He co-starred in the controversial film Downloading Nancy, which was released on 5 June 2009. At the Sundance Film Festival in 2008, audiences walked out of the screening, and as of summer 2008, any theatrical release was still uncertain. Despite the controversy, Sewell continues to staunchly support the film. "It's a film I'm very proud of, whether you consider that it fails or succeeds, whether you like it or don't like it. I'm proud to be in it".
Although best known for his work in costume dramas, Sewell prefers "cravat-less" roles in modern pieces, such as the role of Petruchio in the BBC's 2005 version of Shakespeare's The Taming of the Shrew. This was shown as part of the ShakespeaRe-Told series, and the role earned him a Best Actor nomination at the 2006 BAFTA Television Awards. In this modern retelling of the story, the action moves from 17th century Padua, Italy to 21st century London. This production marked the fourth time that Sewell had acted in a work based on a Shakespeare play since becoming a professional actor: he previously portrayed Hotspur in Henry IV, Part 1 in 1995, Fortinbras in Hamlet in 1996 and the title role in Macbeth in 1999. The role also reunited him with his Charles II co-star Shirley Henderson.
He appeared in the premiere and first run of Tom Stoppard's latest play Rock 'n' Roll at the Royal Court Theatre from June to July 2006 and at the Duke of York's Theatre from July until November 2006. The play was a critical and commercial success, playing to full houses and collecting several awards and nominations, including wins for Sewell in the Best Actor category at The Evening Standard Awards, The Critics' Circle Awards and The Olivier Awards.
He has recorded eleven of Ian Fleming's James Bond books on 36 CDs for Collins. He continues to work in film, television and theatre, playing the lead role of Dr. Jacob Hood in the CBS TV series Eleventh Hour. He finished filming in November 2009 for a miniseries The Pillars of the Earth, which was shown on TV in 2010. In 2010, he played the Italian detective Aurelio Zen, based on the best-selling novels by Michael Dibdin, for the BBC One drama series Zen. The three episodes were filmed in Rome and shown on BBC One in early January 2011. The series was cancelled by the BBC after just one season. He also had a small part in the film The Tourist, which also starred Angelina Jolie and Johnny Depp and was released in cinemas in 2010. He played the lead vampire, Adam, in the film Abraham Lincoln: Vampire Hunter, which was filmed in New Orleans and released in June 2012.
Sewell played the role of Ethics Man in Darkside, Tom Stoppard's 2013 radio drama based on Pink Floyd's album The Dark Side of the Moon. Sewell co-starred with Dwayne Johnson, as Autolycus, in the film Hercules, which was released in July 2014. In 2015, Sewell voiced the role of Sir Claude, a feral cat, in Blinky Bill the Movie. Sewell has most recently appeared as Obergruppenführer Smith in The Man in the High Castle, and as Lord Melbourne in Victoria.
Sewell has been married twice. His first wife was Australian fashion journalist Yasmin Abdallah; they married in 1999 and separated a few months later. He and second wife, scriptwriter and producer Amy Gardner, whom he married in 2004, have a son, William Douglas Sewell (born 2002). They separated in 2006 and subsequently divorced. He also has a daughter.
Sewell has said: "My favourite things are just wandering from place to place, going to cafés, taking photographs. My favourite day is a happy accident".