She was born Geraldine McKeown on 9 May 1932, in Old Windsor, Berkshire, England, daughter of Donald McKeown and his wife Nora (Burns). She was married to the former Principal of RADA, Hugh Cruttwell, who died in 2002.

McEwan attended Windsor County Girls' School and her extensive theatrical career began at 14 as assistant stage manager at the Theatre Royal, Windsor. She made her first appearance on the Windsor stage in October 1946, as an attendant of Hippolyta, in A Midsummer Night's Dream, and played many parts with the Windsor Repertory Company from March 1949 to March 1951 including a role in the Ruth Gordon bio play Years Ago opposite guest player John Clark. She made her first West End appearance at the Vaudeville Theatre on 4 April 1951 as Christina Deed in Who Goes There! in which she made a marked success.

Her career blossomed, with memorable performances in such award-winning productions as The Rivals, The Way of the World, and The Chairs. she also appeared, with Kenneth Williams in the largely disastrous original 1965 production of Loot by Joe Orton, which closed at the Wimbledon Theatre before reaching London.

She spent several seasons with the Royal Shakespeare Company in the late 1950s and early 1960s, where she played in many of the Shakespeare comedies; her roles included an innovative rendition of Olivia in Twelfth Night in 1958.

McEwan worked more than once with Sir Laurence Olivier on both stage and screen, most notably in The Dance of Death staged by Glen Byam Shaw for the National Theatre at the Old Vic in February 1967, one of Olivier's most unsparing and greatest performances.

She made her directing debut in 1988 with the Renaissance Theatre Company's touring season, Renaissance Shakespeare on the Road, co-produced with the Birmingham Rep, and ending with a three month repertory programme at the Phoenix Theatre in London.

McEwan's contribution was a light romantic staging of As You Like It, with Kenneth Branagh playing Touchstone as an Edwardian music hall comedian. In the same season Judi Dench and Derek Jacobi also made their debuts as directors.

Her numerous television credits include the highly acclaimed The Barchester Chronicles with Alan Rickman, The Prime of Miss Jean Brodie, Oranges Are Not the Only Fruit, Mulberry, and the immensely popular Mapp and Lucia.

She was also in an episode of Red Dwarf, playing a computer that can predict the future. In 2002, she starred in Peter Mullan's The Magdalene Sisters, giving an unforgettable performance as Sister Bridget.

In 2004, McEwan was selected by Granada Television as the new face of Miss Marple, the Agatha Christie sleuth, for the series Marple. An impressive critical response and high ratings worldwide for the four episodes transmitted in the UK, Canada and the U.S., secured a second series. McEwan retired from the role after filming the third series, and was replaced by Julia McKenzie.

In 2005, she provided the voice of Miss Thripp in the film Wallace & Gromit: The Curse of the Were-Rabbit.