Raffey began acting aged 7 with roles in the BBC series
32 Brinkburn Street (2011) and ITV's
Mr Selfridge (2013) starring
Jeremy Piven. At aged 11 she was the youngest ever actor to be named in Screen International's Stars of Tomorrow. After early film roles in
Tim Burton's
Dark Shadows (2012) and then
Snow White and the Huntsman (2012), Cassidy went on to star opposite
George Clooney in
Brad Bird's Disney tent pole
Tomorrowland (2015), winning critical acclaim and Best Leading Young Actress and Best Youth Performance nominations (Young Entertainer Awards and Film Critics Association Awards) for her performance as child animatronic Athena. Next up was
Yorgos Lanthimos's Cannes Film Festival Palm d'Or Contender and Best Screenplay winner
The Killing of a Sacred Deer (2017) opposite
Nicole Kidman and
Colin Farrell. Developing a reputation for challenging roles, Cassidy next appeared in
Brady Corbet's
Vox Lux (2018), taking on dual roles opposite
Natalie Portman and
Jude Law and earning a Young British/Irish Performer of the Year nomination at the London Critics Circle Film Awards. After starring in
Malgorzata Szumowska's English language debut
The Other Lamb (2019), which tells the story of an all-female cult, Cassidy next appeared in
Noah Baumbach's adaptation of the Don DeLillo novel
White Noise (2022) alongside
Adam Driver,
Greta Gerwig, and
Don Cheadle. A second collaboration with filmmaker
Brady Corbet followed, starring in
The Brutalist (2024) opposite
Adrien Brody,
Guy Pearce, and
Felicity Jones.