She followed this with a dual role in director Brady Corbet's Vox Lux (2018) and her first top billing in The Other Lamb (2019).

[2][3] Her father teaches acting, and she grew up in a family where her siblings were also involved in the entertainment business.

[4][5] Her first part came unexpectedly as she was accompanying her older brother, Finney Cassidy on an audition for a role on a BBC drama.

[6] Cassidy's credits in 2012 include brief film roles in Warner Bros' Dark Shadows, directed by Tim Burton and starring Johnny Depp, as well as Universal Pictures' Snow White and the Huntsman, opposite Charlize Theron, Kristen Stewart, and Chris Hemsworth.

In 2015, Cassidy completed production as Molly in Amber Entertainment's Molly Moon and the Incredible Book of Hypnotism, with Emily Watson and Dominic Monaghan, and starred as the Audio-Animatronic Athena in Disney's Tomorrowland, with George Clooney, Hugh Laurie, and Britt Robertson.