In the summer of 2007, Casey played Serena Katz in a hit run of the musical Fame at the Shaftesbury Theatre, and in February 2008 she began playing the role Julia in the pre-West End UK tour of The Wedding Singer. Her other stage credits include The Vagina Monologues (Palace Theatre, Manchester), Hobson's Choice (Watermill Theatre opposite her sister Anna-Jane Casey), Flint Street Nativity (Liverpool Playhouse), Well at the Trafalgar Studios, and the Apollo Theatre, opposite Sarah Miles and most recently Ado Annie in Oklahoma! at the Chichester Festival Theatre. She took over from Denise Van Outen in Legally Blonde at The Savoy Theatre as Paulette from 26 April 2011. In March 2012 she played Angela in the revival of Mike Leigh's Abigail's Party at the Menier Chocolate Factory, later transferring to Wyndhams Theatre, London.