Jessica Ellen Cornish was born in London,to Rose (née Archer) and Stephen Cornish. She was educated at Mayfield High School in the London Borough of Redbridge. She attended Colin's Performing Arts School and as an 11-year-old she was cast in Andrew Lloyd Webber's West End production of Whistle Down the Wind.She subsequently joined the National Youth Music Theatre and appeared in their 2002 production of The Late Sleepers.