Thomasin's first acting job was in the New Zealand feature film EXISTENCE at age 7, but her first big role was at age 13, when she played the young Louise Nicholas in the New Zealand drama CONSENT. From there, Thomasin went on to have roles in Shortland Street, Sunday Theatre dramas and web series, notably Lucy Lewis Can't Lose.

In 2017, whilst in Year 12, Thomasin headed to America to play the daughter of a war veteran suffering PTSD. She won critical acclaim for her portrayal of Tom in LEAVE NO TRACE. Thomasin was recognised by the National Board of Review and was given the award for Breakout Performance in 2018, amongst many other awards that year.

She attended the Cannes Film Festival, then travelled to Prague in mid-2018 to begin work on Taika Waititi's JOJO RABBIT. Whilst there, Thomasin also filmed scenes for Netflix's THE KING with Timothee Chalamet in England, before heading to Australia for the TRUE HISTORY OF THE KELLY GANG. At the end of Year 13, Thomasin filmed LOST GIRLS in New York, a true crime story for Netflix.

In her first year out of school, Thomasin spent time in America promoting her work, before basing herself in London to play the lead role in film LAST NIGHT IN SOHO. She then returned to New Zealand to film the feature THE JUSTICE OF BUNNY KING. JOJO RABBIT was released in October 2019, and resulted in an invitation to the Golden Globes in January 2020. Thomasin won the Next Generation of Hollywood award from the Hollywood Critics Association and the Best Youth Performance from the Seattle Film Critics Association. JOJO RABBIT was nominated for 6 Oscars, including Best Picture, and won for Best Adapted Screenplay.

In 2020, Thomasin was invited to join the Academy of Motion Pictures, which allows her to vote at the Oscars. Since then she has appeared in LAST NIGHT IN SOHO, THE POWER OF THE DOG, OLD as well as the limited BBC series LIFE AFTER LIFE. Th the last couple of years she has released EILEEN, TOTALLY COMPLETELY FINE and JOY.