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.