Lewis was born in the London district of Islington to parents Aural Josiah, a Guyanese Afro-Caribbean youth worker, and Marie Lewis, a social worker of Anglo-Welsh descent.[5][6] At the age of five she attended the Sylvia Young Theatre School,[6] and later the Italia Conti Academy[7] and the BRIT School[8]

Lewis wrote her first song at the age of twelve,[9] and won a number of talent competitions.[10][11] After leaving school, Lewis took a number of jobs to pay to get into a recording studio, including waitressing and several receptionist jobs.[6][7][12]

She recorded a number of songs including a demo album of her own songs called Twilight, but the album was never released.[13] When she was fifteen years old she recorded a version of Minnie Riperton's "Lovin' You" with music producer Marley J. Wills, after which Lewis and Wills were invited to the U.S. by Sony.[14] Wills commented that "Leona could hit the notes and I think she did it better than the original."[15]

When her music career seemed to be going nowhere, she decided to give up, until her boyfriend persuaded her to enter The X Factor