Harris was born and raised in London, England, the daughter of sitcom writer Lisselle Kayla, a single mother who had immigrated to the UK from Jamaica as a child.

After graduating in 1998 from Pembroke College, Cambridge with an undergraduate degree in social and political science, Harris trained at the Bristol Old Vic Theater School.

Harris is a Buddhist. She is was recently photographed out on a date with Pirates of the Caribbean co-star Orlando Bloom, though it has not been confirmed by either party whether the two are officially an item.

A television and film actress since she was nine, Harris only became well-known in 2002 with her lead performance in Danny Boyle's post-apocalyptic film 28 Days Later..., opposite Cillian Murphy. In the same year, she starred in the television adaptation of Zadie Smith's White Teeth.

Since then, Harris has been cast primarily in supporting roles in big budget films, such as Pirates of the Caribbean: Dead Man's Chest, Pirates of the Caribbean: At World's End and Michael Mann's Miami Vice. She also did a comic turn in Michael Winterbottom's well-regarded indie ensemble piece, Tristram Shandy: A Cock and Bull Story, and will appear with Josh Hartnett in the 2008 release, August.