Thomasin started getting major acting roles in her teens, but she had been exposed to acting and filmmaking from a very young age. Her mother, Miranda Harcourt, is an actress and acting coach; her grandmother Dame Kate Harcourt is an actress; and her father, Stuart McKenzie, is a local director.
"I'm a third-generation actress," she said. "[Acting] is just something that I've learned kind of just being around my family and learning a lot through osmosis."