Smith was born in Ilford in Essex, the daughter of Nathaniel Smith, a Newcastle-born public health pathologist who worked at Oxford University, and Margaret Hutton Little, who was Scottish. She has two older twin brothers, Alistair and Ian. Smith studied at Oxford High School although she has been quoted as having not enjoyed the experience, at a time when the likes of Lady Antonia Fraser would have been amongst her peers.

Smith has had an extensive career on both screen and in live theatre and is known as one of Britain's pre-eminent actresses. She started her career at the Oxford Playhouse with Frank Shelley, and made her first film in 1956. She became a fixture at the Royal National Theatre in the 1960s, most notably for playing Desdemona in Othello opposite Laurence Olivier, and winning her first Oscar nomination when she immortalized her performance in the 1965 film version. In 1969 she won the Academy Award for Best Actress for her performance as an unorthodox Scottish schoolteacher in The Prime of Miss Jean Brodie, a role originally created on stage by Vanessa Redgrave in 1966. She was also awarded the 1978 Academy Award for Best Supporting Actress for her role as the brittle actress, Diana Barrie, in California Suite acting opposite Michael Caine. Afterwards, Caine is supposed to have humorously telephoned Michael Palin on hearing that Palin was about to embark on a film (The Missionary) with Smith, warning him she would steal the film .

Smith appeared in Sister Act in 1992, and had a major role in the 1999 film Tea With Mussolini where she appeared as the formidable Lady Hester. Indeed, many of her more mature roles have centred on what Smith self-mockingly refers to as her "gallery of grotesques", and indeed both her directors and her audiences love to see her playing waspish, sarcastic or plain rude characters; it is to her credit that she bestows such unsympathetic roles with a humanity and vulnerability which lesser actors could not. Recent examples of this would include the judgemental sister in Ladies in Lavender and the cantankerous snob in Gosford Park for which she received yet another Oscar nomination.

Other notable roles include the querulous Charlotte Bartlett in the Merchant-Ivory production of A Room with a View and a vivid supporting turn as the aged Duchess of York in Ian McKellen's film of Richard III. Given the international success of the Harry Potter movies, she is widely known in the role of Professor Minerva McGonagall in the Harry Potter films. She is due to appear in Harry Potter and The Order of the Phoenix in July 2007.

Throughout her career, Smith has been admired for her remarkable technique, on both stage and screen. She has the ability to project a quality of deep emotion whether comic or tragic, balanced by an innate reserve that combines the appearance of steely control and a hint of something approaching hysteria. Off stage, however, she is sometimes perceived as a reserved and private person. To her legion of devoted and sometimes fanatical admirers, however, she is one of the great actresses of film and theatre with an idiosyncratic style quite unlike anyone else; it was during the 1970s that she moved to Canada to find a new direction in both her art and in her personal life as she had recently become divorced.

On stage, her many roles include the title character in the stage production of Alan Bennett's Lady in the Van and starring as Peter Pan[citation needed] in J. M. Barrie's fairytale story Peter Pan. She later played Wendy in the Peter Pan adaption Hook. She won a Tony Award in 1990 for Best Actress in a Play for Lettice and Lovage, starring as an eccentric tour guide in an English stately home.

She was awarded Commander of the Order of the British Empire (CBE) in 1970, and was raised to Dame Commander (DBE) in 1990.

Smith has been married twice. She married Robert Stephens on 29 June 1967, at the Greenwich Registry office and had two sons with him: actors Chris Larkin (born 1967) and Toby Stephens (born 1969). They divorced on 6 May 1974.

She married Beverly Cross (on 23 August 1975 at Guildford Registry Office) and the marriage ended with his death on 20 March 1998. At the time of his death she was appearing in A Delicate Balance at the Theatre Royal, Haymarket yet with characteristic fortitude she continued to the end of the run.[citation needed]

She has been nominated twice for the Academy Award for Best Actress in a Leading Role, winning it once:

She has been nominated four times for the Academy Award for Best Actress in a Supporting Role, winning it once:

Smith has won the BAFTA Award for Best Actress in a Leading Role four times: