Famous for her role as Phyllis Waterbury in the 1970 English movie The Railway Children and for her portrayal of Janice Hedden in the 1971 psychological film Straw Dogs, she is also notable for her performance as Jo in the 1970s situation comedy Man About the House.
Early in her career, she appeared in the 1960s films Seventy Deadly Pills and Danny the Dragon.
She was nominated for a BAFTA Award in 1970.
After the conclusion of a three-year relationship with fellow actor Richard O'Sullivan, she married Paul Agnew and gave birth to a daughter named Charlotte.
Despite being older than her co-star Jennifer Agutter, Thomsett played Agutter's character's younger sister in The Railway Children.