Beginning as a child actor, Le Touzel's first television role was in the Doctor Who story The Mind Robber, playing one of the children who bedevil the Second Doctor and his companions in the Land of Fiction. She co-starred in the BBC Schools "Look and Read" series, playing Helen in their cult serial The Boy from Space (1971), which was re-edited with a new introduction in 1980. A well-known early adult role was as Fanny Price in the BBC dramatisation of Jane Austen's Mansfield Park (1983).
She also appeared in a classic Heineken advert "The Water in Majorca" with Bryan Pringle. This advert, first aired in 1985, was ranked at number 9 in Campaign Live's 2008 list of the "Top 10 Funniest TV Ads of All Time", and at number 29 in Channel 4's list of the "100 Greatest TV Ads" in 2000.
Le Touzel has also been seen on the small screen in shows as diverse as Dixon of Dock Green, The Brontes of Haworth, The Uninvited, The Professionals, Lovejoy, Alas Smith and Jones, Midsomer Murders, My Family and The Long Walk to Finchley (as Patricia Hornsby-Smith). In 1994 she played Sarah Teale in the TV series Between the Lines. In 2000 Sylvestra was cast as a veterinary surgeon in the sitcom Beast, written by Simon Nye, which also starred Alexander Armstrong. Only 12 episodes were made over two series. In 2007, she also played the role of Mrs Allen in the ITV adaptation of Northanger Abbey. She has also played Harriet Waterhouse in the ITV drama Bonkers.
Le Touzel married the Welsh actor Owen Teale in 2001, several years after they had met during a production of King Henry IV in 1991, and they have two children together. Le Touzel is a supporter of charitable theatre and in 2005 became a patron of Montage Theatre Arts, London, along with fellow Actress Debby Bishop.
Le Touzel played Marianne Thornton in the 2006 film Amazing Grace and appeared in Happy-Go-Lucky.
In 2011 she starred as Detective Constable Hazel Savage in Appropriate Adult.
In 2012 film Cloud Atlas, she portrayed three different characters through the movie: Haskell Moore's Dinner Guest 5, Nurse Judd and Aide in Slaughtership.
In 2015 Le Touzel appeared as Sandra Kendrick in the BBC TV series Death in Paradise episode 4.7
In 2017, she portrayed Lady Dorothy Macmillan in the Netflix series The Crown.
