Thomasin Harcourt McKenzie was born in Wellington, New Zealand, to actress Miranda Harcourt and director Stuart McKenzie. She is the granddaughter of actress Dame Kate Harcourt and Peter Harcourt. Peter's family founded the real estate company Harcourts International in Wellington. She has an elder brother and a younger sister, actress Davida McKenzie. She completed her secondary education at Samuel Marsden Collegiate School in 2018. After a good role in The Hobbit: The Battle of Five Armies, she rose to critical prominence with the lead role in Debra Granik's 2018 drama film Leave No Trace. After supporting roles in the 2019 films The King, Jojo Rabbit, and True History of the Kelly Gang, as well as the 2021 thriller Old, she played the lead role as Eloise, a wide-eyed woman from Cornwall in Edgar Wright's psychological horror film Last Night in Soho.