Tyzack was born in Essex, England, the daughter of Doris (née Moseley) and Thomas Edward Tyzack. She grew up in West Ham (now Greater London). She attended the all-girls' St Angela's Ursuline School, Newham, and was a graduate of RADA. Her television roles included The Forsyte Saga (1967) and I, Claudius (1976). She won the 1970 BAFTA TV Award for Best Actress for the BBC serial The First Churchills, and the 1990 Tony Award for Best Featured Actress in a Play for Lettice and Lovage, opposite Maggie Smith. She also won two Olivier Awards. Her film appearances included 2001: A Space Odyssey (1968), A Clockwork Orange (1971), Prick Up Your Ears (1987) and Match Point (2005).
Tyzack was appointed Officer of the Order of the British Empire (OBE) in the 1970 Birthday Honours and Commander of the Order of the British Empire (CBE) in the 2010 New Year Honours, both for services to drama.
Tyzack married mathematician Alan Stephenson in 1958 and together they had one son, Matthew. She died on 25 June 2011 after a short illness. She died at her home with her family by her side. Her family told the Daily Mail that she had faced her illness with "the strength, courage, dignity and even humour with which she lived her life."