Harewood was born in the Small Heath area of Birmingham on 8 December 1965, the son of a couple from Barbados who had moved to England in the late 1950s and early 1960s. His father was a lorry driver, while his mother was a caterer. He has a sister, Sandra, and two brothers, Rodger and Paul. He attended St Benedict’s Junior School and Washwood Heath Academy. He was a member of the National Youth Theatre. In his youth, he worked at a wine bar in Birmingham city centre. At the age of 18, he gained a place at the Royal Academy of Dramatic Art.

Harewood began acting in 1990 and has appeared in The Hawk, Great Moments in Aviation, Harnessing Peacocks, Mad Dogs and Englishmen, Blood Diamond, The Merchant of Venice and Strings. He is known for his television appearances on Ballykissangel, The Vice and Fat Friends. He played Don Coleman in Hustle (Series 7 The Fall of Railton FC (2011)). In 1997, he was the first black actor to play Othello at the National Theatre in London.

Harewood played Patroclus in the 1998 BBC radio trilogy Troy. He also played Henry Tilney in Northanger Abbey radio adaptation (2005). On 4 May 2012, he hosted a special BBC Radio 2 Friday Night is Music Night celebrating the life of Ray Charles, broadcast live from Cheltenham Jazz Festival. The show featured the Guy Barker orchestra, with leader Cynthia Fleming and guest artists Madeline Bell, Gregory Porter, and James Tormé.

In 2007, Harewood visited Harewood House in Yorkshire and spoke with Viscount Lascelles who is a cousin of the Queen. His surname comes from the time when his ancestors were sold in Africa, transported to the Caribbean as slaves, and forced to work for the Lascelles family (the Earls of Harewood). Lord Lascelles explained that his wish was for the Harewood name to stand for positive things in the future, as nothing could be done about what happened 250 years ago.

In 2007, Harewood donated his bone marrow and as a result saved the life of a patient.

In 2008, he played Major Simon Brooks in The Palace; he also appeared (that December) on Celebrity Mastermind, with specialist subject Philip Pullman’s His Dark Materials – and he appeared in the BBC film adaptation of the Philip Pullman novels The Ruby in the Smoke and The Shadow in the North, both of which are titles from the Sally Lockhart Mysteries.

In 2009, Harewood appeared in the BBC single drama Mrs Mandela, playing Nelson Mandela. He played Brother Tuck in the third series of Robin Hood. He appeared in the Doctor Who story “The End of Time”. He played Martin Luther King Jr. in the premiere of The Mountaintop, written by American playwright Katori Hall, directed by James Dacre, which opened at Theatre503 in London on 9 June 2009.

From 2011, Harewood starred as David Estes, the director of the CIA’s Counterterrorism Center, in the Showtime series Homeland. After appearing in 24 episodes, his character was killed off in a bomb explosion at the end of season 2. Also in 2011, he voiced Captain Quinton Cole in the video game Battlefield 3.

In the 2012 New Year Honours, Harewood was appointed Member of the Order of the British Empire (MBE) for services to drama. In May 2012, he presented a Party Election Broadcast for the British Labour Party.

In October 2013, Harewood voiced an interactive video campaign for the British Lung Foundation aiming to ban smoking in cars with children on board in the United Kingdom. In June 2014, he appeared in Tulip Fever.

Harewood married his long-term girlfriend Kirsty Handy in February 2013 in Saint James, Barbados. They have two daughters and the family reside in Streatham, London. Harewood is an avid supporter of Birmingham City.

In October 2015, he appeared as a core cast member on the CBS television series Supergirl as Hank Henshaw. Since his character was revealed (in the episode Human for a Day) to be J’onn J’onzz/Martian Manhunter posing as Henshaw, he portrays J’onn J’onzz with Henshaw’s likeness as his human form and has a dual recurring role as the real Hank Henshaw / Cyborg Superman.

Harewood was included in the 2019 edition of the Powerlist, ranking the 100 most influential Black Britons. Also in 2019, he played the position of goalkeeper for England in Soccer Aid for UNICEF 2019. Pyschosis and Me, a documentary hosted and produced by Harewood received a BAFTA Television Award nominated for Single Documentary.

In the 2019 European Parliament election, Harewood pledged his support for Change UK.