Her films include The Sun, the Moon and the Stars (1996), Felicia's Journey (1999), The Others (2001), Disco Pigs (2002), And When Did You Last See Your Father?

[2] In 1996, Cassidy was nominated for the Most Promising Actress at the Geneva Film Festival for her role in The Sun, the Moon and the Stars.

In 2015, she played a main role of DC Dinah Kowalska in the Channel 4 drama series No Offence (2015–2018),[2] and Sarah Manning in Acceptable Risk (2017).

[7] In 2022, she starred as Rosaleen O'Donnell in the Irish film The Wonder, alongside her own daughter Kila, who played "the wonder" and co-starring Florence Pugh.

[10] Cassidy married English actor Stephen Lord, whom she met on the set of The Truth, on 31 December 2007.