Annis was born in Rio de Janeiro to a British father, Lester William Anthony Annis (1914-2001) and a Brazilian-French mother, Mariquita Purcell (1913-2009). She has two brothers, Quenton D. and Tony P. Annis. Her family moved to England when she was 7. There she was educated at a convent school. She trained as a ballet dancer and then studied drama at the Corona Stage Academy.

Annis began acting professionally in her teens, and made her film debut in the 1950s. In 1967 she played Estella in a television adaptation of Great Expectations. She also presented children's television programmes.

While becoming an increasingly well-known face on screen, she has enjoyed a successful stage career, playing many leading roles with the Royal Shakespeare Company in productions such as The Comedy of Errors. At the National Theatre in 1981 she played Natalya Petrovna in Peter Gill's production of Ivan Turgenev's A Month in the Country.

Annis gained notoriety for her performance as Lady Macbeth in Roman Polanski's controversial 1971 film interpretation of Macbeth in which she recites the sleepwalking soliloquy in the nude.

She continued to be a leading television actress throughout the 1970s, 1980s and 1990s, appearing in series such as Edward the Seventh as Lillie Langtry, a role she starred in with Lillie; Madame Bovary; and Parnell and the Englishwoman, in which she played Kitty O'Shea; as well as the miniseries Reckless (1998) and its 2000 sequel.

Her films have included Krull (1983) and Dune (1984). She appeared as "Tuppence" with James Warwick as "Tommy" in Agatha Christie's Partners in Crime stories. She also appeared in Onassis: The Richest Man in the World, in which she played Jacqueline Kennedy.

At the Comedy Theatre in September 2005, Annis starred in Epitaph for George Dillon with Joseph Fiennes. Annis returned to the stage in April 2009 to star as Mrs. Conway in Rupert Goold's National Theatre revival of J B Priestley's Time and the Conways .

In autumn 2007 Annis co-starred with Michael Gambon and Judi Dench as Lady Ludlow (an aristocrat opposed to the education of the lower classes) in the BBC1 costume-drama series Cranford.

Annis has three children from a long-term relationship with Patrick Wiseman that began in 1976.

Beginning in 1994, Annis was in a long relationship with actor Ralph Fiennes, who left his younger wife, Alex Kingston. They met while performing Hamlet, in which Annis portrayed Gertrude with Fiennes' playing Hamlet. On February 7, 2006, Fiennes and Annis announced their separation after 11 years together.