Fionn was born in Richmond, London to Linda and acclaimed British jazz musician Tim Whitehead. Growing up, Fionn wanted to be a breakdancer, but changed his mind and decided he would like to pursue a career in acting instead.
At the age of 13, Fionn started acting at the Orange Tree Theater, and then went to Orleans Park School. After graduating from secondary school, Fionn attended Richmond College and entered in the National Youth Theatre's summer course.
In 2016, Fionn made his television debut, starring on the miniseries Him (2016), playing the title role of Him. In the same year, he acted in a stage performance of Glenn Waldron's Natives, and was recognized to be an up-and-coming actor, as he was included in the 13th edition of Screen International's Stars of Tomorrow.
Fionn got his big break in 2017, when he starred as Tommy in Christopher Nolan's Dunkirk, opposite Tom Hardy, Harry Styles, and Cillian Murphy. Along with the film's critical acclaim and numerous awards and nominations, Fionn's work as Tommy also earned him nominations for the Empire Award for Best Male Newcomer and the London Film Critics' Circle for Young British/Irish Performer of the Year.
In 2018, Fionn starred in the interactive sci-fi film, Black Mirror: Bandersnatch (2018), playing Stefan Butler.
Most recently Fionn played the role of Zac in the film Voyagers (2021). He's currently working on the film Emily, about Emily Brontë, playing her brother Bramwell.
Fionn, who was named after Irish folk legend Fionn mac Cumhaill, has three siblings — two older sisters Maisie and Hattie, and a younger brother, Sonny. Maisie, the eldest, is a physical performer and theater maker, whereas Hattie is a folk artist. Fionn appeared in the music video for Hattie's song "Ups and Downs." ~Sean Olegario
Don’t Tell a Soul (2020)
The Duke (2020)
Port Authority (2019)
Black Mirror: Bandersnatch (2018)
The Children Act (2017)