Born in Toronto, Ontario, Canada, she attended Bishop Strachan School, and then moved to the UK in 1965 to study acting. She graduated from the Royal Academy of Dramatic Art with an Honours Diploma, including speaking and singing honors (soprano), on 1 July 1967. Her professional name is based upon her first married name, Mrs Barry Bergthorson.
Thorson is best known for her role as Tara King (succeeding Diana Rigg as Emma Peel) in the last season of the British TV adventure series, The Avengers, with the original star Patrick Macnee. She was reunited with Macnee in a commercial for Laurent-Perrier champagne in the mid 1970s which led to the series reappearing as The New Avengers, although Thorson did not regain her role.
Thorson has performed in many dramatic and musical stage productions, including appearances on Broadway in Nell Dunn's Steaming and Michael Frayn's Noises Off!. In 1971, she starred alongside Michael Crawford and Anthony Valentine in the London West End hit show No Sex Please, We're British and later appeared as Titania in A Midsummer Night's Dream at the Open Air Theatre, Regents Park, London. She also played Hester Salomon in a UK tour of Equus opposite Simon Callow. In the summer of 2008, she appeared at the Open Air Theatre, Regent's Park in the Lerner & Loewe musical, Gigi.
She appeared from 1989 to 1992 in the daytime drama One Life to Live as Julia Wheaton Medina. She also appeared in Star Trek: The Next Generation, playing female Cardassian starship commander Gul Ocett in the 6th season episode 'The Chase' (1993). From 1998-2000, she played Isabel in the Canadian series Emily of New Moon.
In 2002, she portrayed a Supreme Court Justice in the movie Half Past Dead with Steven Seagal and Ja Rule, and appeared in the 2006 action sequel Max Havoc: Ring of Fire. Throughout 2006-07, Thorson played the villainous Rosemary King in the ITV series Emmerdale.
In 2013, Thorson produced a new stage play called The Goodbye Bird written by Colleen Murphy.
