Warner was born Amelia Catherine Bennett in Birkenhead, Merseyside, the only child of actors Annette Ekblom and Alun Lewis.
She also starred in a 2000 BBC adaptation of Lorna Doone and has had supporting roles in films such as Æon Flux and Stoned.
On 28 September 2020, Fearne Cotton announced that she was releasing "Happy Place", the album which features music from an array of artists that focus around mental health and well-being.
Warner began her music career in July 2009, when she wrote and performed the song "Beasts" under the name Slow Moving Millie for a Virgin Media television commercial.
Her cover version of the Smiths' 1984 B-Side "Please, Please, Please, Let Me Get What I Want" was released on 11 November 2011 and was selected as the soundtrack to the John Lewis 2011 Christmas advertisement.