Warner was born in Liverpool, Merseyside, the only child of actress Annette Ekblom and Welsh-born actor Alun Lewis. Her paternal uncle is actor Hywel Bennett. When her parents divorced, Warner, then four years old, and her mother moved to London, England. Warner studied at the Royal Masonic School for Girls and, at 16, the College of Fine Arts in London. She studied history of art at Goldsmiths College in London. She got her start in acting as a member of the Royal Court's youth theatre group. She starred in a 2000 BBC adaptation of Lorna Doone and has had supporting roles in recent films such as Æon Flux and Stoned.