Casey was born in Rawtenstall, Lancashire. At the age of three her single "Chick Chick Chicken" reached number 72 in the UK Singles Chart in 1984. This made her the youngest person ever to have a recording in the United Kingdom chart. At this time, she appeared on the BBC's Saturday Superstore and asked Boy George to take her to the toilet. In the 2008 live special of Two Pints of Lager and a Packet of Crisps, Casey performed the single for the first time in over 20 years.

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.

On 29 June 2012, it was confirmed that Casey would be playing the role of Judy Bernly in the first UK tour production of the Broadway musical 9 to 5. The tour began at the Manchester Opera House on 12 October 2012. She appeared in the UK tour production of 9 to 5 in a cast which includes Bonnie Langford and Jackie Clune. She appears in 'Sex and the Three Day Week'.

Casey has also presented and narrated numerous shows such as Dinner Date from 2014. She co-presented Big Brothers Little Brother in 2001 with Dermot O' Leary.