She was born Julia Kathleen McKenzie on 17 February 1941, in Enfield, Middlesex, England, the daughter of Kathleen Rowe and Albion McKenzie.
In London's West End her performing credits include Guys and Dolls as Miss Adelaide (1982) and Sweeney Todd as Mrs. Lovett (1994), winning the Olivier Award for Actress of the Year in a Musical for each.
For her role in Woman in Mind, she received the Evening Standard Award for Best Actress. She has also appeared in Follies as Sally at the Shaftesbury Theatre in 1987 and Into the Woods as the Witch at the Phoenix Theatre in 1990. She appeared in Side By Side By Sondheim in the West End in 1976 and on Broadway in 1977, and was nominated for a Tony Award and a Drama Desk Award for her performance.
McKenzie appeared in a National Theatre 80th birthday tribute to Lord Olivier, Happy Birthday, Sir Larry on 31 May 1987 in the presence of Olivier himself.
On television, McKenzie co-starred with Irene Handl in the sitcom Maggie and Her (1978–79), and with Gareth Hunt in That Beryl Marston...! (1981). She went on to greater popularity with British viewers as Hester in Fresh Fields and the sequel French Fields in the 1980s opposite Anton Rodgers, for which she was voted TV Times Favourite Female Comedy Performance for five consecutive years. She also appeared as Mrs Forthby in Blott on the Landscape and as a Midsomer villager involved in a series of murders in an episode of Midsomer Murders. Film credits include Hotel du Lac, Shirley Valentine, Bright Young Things and These Foolish Things.
In 2007 she was reunited with Anton Rodgers (again as a husband and wife team) in the ITV comedy You Can Choose Your Friends. In 2007 she co-starred with Michael Gambon and Judi Dench in the BBC1 costume drama series Cranford, playing Mrs. Forrester, a military widow of slender means, very attached to her cow Bessie.
In 2008 she was announced as the replacement for Geraldine McEwan as ITV's Miss Marple. McKenzie noted: "It’s difficult because Agatha Christie wrote her in two ways...First, very much what Geraldine McEwan played: a slight, rather Victorian creature. Then, a little sturdier and tweedier. I chose the latter. A lot of people say they don’t like the tweedier version. But they’re both genuine." Also, she said: "Just about everybody in the world knows about Miss Marple and has an opinion of what she should be like, so I’m under no illusions about the size of the task ahead." McKenzie's first series of Marple comprises A Pocket Full of Rye, Murder is Easy, Why Didn't They Ask Evans?, and They Do It with Mirrors. The second series of the show, airing in 2010, included The Mirror Crack'd from Side to Side, The Secret of Chimneys, The Blue Geranium, and The Pale Horse. A sixth series, including adaptations of A Caribbean Mystery, the short story Greenshaw's Folly and Endless Night, is currently being aired.
During the 2012 Summer Olympics opening ceremony she played Her Majesty the Queen on board the helicopter in the short film Happy and Glorious.
She is a radio performer with a long list of credits, including Blithe Spirit, The Country Wife and A Room with a View. As a director she has staged Stepping Out, Peter Pan, Hey, Mr. Producer!, Steel Magnolias, Putting It Together and A Little Night Music.
She also recorded an audio book of Lewis Carroll's Through the Looking Glass.
McKenzie has been married since 1971 to American actor-director, Jerry Harte.
- (London debut) Maggie May, Adelphi Theatre, 1966
- Gloria, Mame, Drury Lane Theatre, London, 1969
- Girl in owl coat, Promises, Promises, Prince of Wales Theatre, London, 1970
- April, Company, Her Majesty's Theatre, London, 1971
- Cowardy Custard, Mermaid Theatre, London, 1973
- Cole, Mermaid Theatre, 1974
- (New York debut) Side by Side by Sondheim, Music Box Theatre, New York City, 1977
- Norman Conquests, 1978
- Ten Times Table, 1979
- Miriam Dervish, Outside Edge, Queens Theatre, 1979
- Lily, On the Twentieth Century, Her Majesty's Theatre, 1980
- Maggie Hobson, Hobson's Choice, Lyric Hammersmith Theatre, London, 1981
- Anna Kopecka, Schweyk in the Second World War, Olivier/National Theatre, London, 1982
- Miss Adelaide, Guys and Dolls, Olivier/National Theatre, 1982
- Susan, Woman in Mind, Vaudeville Theatre, London, 1986
- Sally Plummer, Follies, Shaftesbury Theatre, London, then West End Theatre, New York City, 1987
- Happy Birthday, Sir Larry, Olivier Theatre, National Theatre, 31 May 1987
- Witch, Into the Woods, Phoenix Theatre, London, 1990
- Sweeney Todd, National Theatre, London, 1994
- Kafka's Dick, Piccadilly Theatre, 1998
- Fuddy Meers, Arts Theatre, 2004
- The Philadelphia Story, Old Vic, 2005
- Stepping Out, Duke of York's Theatre, London, 1984
- Just So, Watermill Theatre, Bagnor, Berkshire, England, 1989
- Steel Magnolias, Lyric Theatre, London, 1989
- Putting It Together, Manhattan Theatre Club Stage I, New York City, 1993
- (With Bob Avian) Hey, Mr. Producer! The Musical World of Cameron Mackintosh, Lyceum Theater, London, 1998
- A Little Night Music, Tokyo, 1999
- Dick Deadeye, or Duty Done (1975)
- The Wildcats of St. Trinian's (1980)
- Hotel du Lac (1986)
- Shirley Valentine (1989)
- Vol-au-vent (1996)
- Bright Young Things (2003)
- These Foolish Things (2006)
- Notes on a Scandal (2006)
- Agatha Christie: The Pale Horse (BBC/PBS/WGBH TV) (2011)