Harris has appeared in television and film since she was nine, including a starring role in the remake of the science fiction series "The Tomorrow People". In 2000, she played Susan in "The Witch of Edmonton" at the Southwark Playhouse. She was in the cast of She Stoops to Conquer (Radio 3), Studio 3, Pebble Mill, May 2000. In November 2002, she starred in Danny Boyle's postapocalyptic film "28 Days Later". In the same year, she starred in the television adaptation of Zadie Smith's "White Teeth". Harris has appeared in ', ', and Michael Mann's "Miami Vice." She did a comic turn in Michael Winterbottom's indie ensemble piece, "A Cock and Bull Story" (2005). She starred in Channel 4's adaptation of the 2006 novel "Poppy Shakespeare", which was first shown on 31 March 2008. She also appeared in BBC's historical drama "Small Island" in December 2009.