Cassidy's most notable role so far is that of Rachel Towers in BBC children's drama Grange Hill, but she has had a number of smaller roles in some of the biggest television series of the past few years, including The Street, The Chase and Marian, Again. Cassidy also had a guest spot in Casualty spin-off drama Casualty 1909 appearing alongside her real-life brother Mossie Cassidy in his first TV role. She is the older sister of Raffey Cassidy.
Cassidy joined Emmerdale in early 2009 as Hannah Barton, with her first appearance in the soap being 17 July 2009. On 28 April 2012, Cassidy confirmed her departure from Emmerdale.