Harris was born on 6 September 1976 in Islington, London, where she was raised. Harris' mother, Lisselle Kayla, emigrated from Jamaica to London as a child with her parents, and her father, a fashion designer, emigrated from Trinidad. They separated before she was born, and Harris was raised by her mother and has no relationship with her father. She grew up in humble beginnings, raised by a single mother in a council flat in Finsbury Park. Her mother later remarried and Harris has two younger half-siblings. Her mother worked as a Screenwriter on EastEnders and is now a healer. Harris attended St Marylebone School in London, before attending Woodhouse College as a 6th form student. She graduated from Pembroke College, Cambridge in 1998 with a degree in social and political sciences. Harris gained her training at the Bristol Old Vic Theatre School.