Marsh was born in Stoke Newington, London, the daughter of Emmeline Susannah Nightingale Poppy (née Bexley), a bar employee and dresser for the theatre, and Henry Charles John Marsh, an outdoor maintenance man and printer's assistant.
During the 1950s and 1960s, Marsh made many appearances on British and American television including an episode of The Twilight Zone called "The Lonely" (1959), in which she played Alicia, a lifelike and attractive female robot; The Moon and Sixpence (1959) opposite Laurence Olivier and Denholm Elliott; The Wonderful World of Disney (1961); Gideon's Way (1965); I Spy (1967); The Saint (4 episodes between 1964 and 1968); and UFO (1970). She was a regular in the ITV series The Informer (1966–67) starring Ian Hendry.
Marsh appeared several times in the BBC series Doctor Who. She first appeared alongside William Hartnell in the 1965 serial The Crusade as Lady Joanna, the sister of Richard I (The Lionheart). She returned later that year as companion Sara Kingdom in the 12-part serial The Daleks' Master Plan. Marsh reprised the role in the audio plays Home Truths (2008), The Drowned World (2009), The Guardian of the Solar System (2010), The Five Companions (2011), The Anachronauts (2012), An Ordinary Life (2014) and The Sontarans (2016). She also appeared in the 1989 television serial Battlefield as Morgana Le Fay, as well as the 2007 audio play The Wishing Beast. She made an un-billed cameo appearance in the 2013 docudrama about Doctor Who, An Adventure in Space and Time.
Marsh was featured as Bertha Mason Rochester in the George C. Scott-Susannah York version of Jane Eyre, directed by Delbert Mann. The film was released theatrically in the United Kingdom in 1970 and shown in the United States on NBC Television in 1971.
Marsh's 2000 film about the death of Henry VIII, Monarch, with T. P. McKenna, written and directed by John Walsh was re-released in cinemas in 2014.
With Eileen Atkins, Marsh created the British period drama Upstairs, Downstairs, and played the role of the house parlourmaid Rose Buck for the duration of the series, from 1971 until 1975. The programme was internationally popular and received numerous awards including two BAFTAs, two Royal Television Society awards, eight Emmys and a Golden Globe. Marsh received a Royal Television Society award in 1971 and an Emmy Award for her role in 1975; and was nominated for the same award on three further occasions – 1974, 1976 and again for the revival in 2011. The actress also received awards from the American Drama Centre and American Drama Critics Circle for the role, and two Golden Globe Award nominations.
Marsh and Eileen Atkins created a second television series The House of Eliott, three series of which were broadcast between 1991 and 1994. This time, Marsh did not act in the series, but she did write some of the episodes.
Her film credits include the Tony Hancock film The Rebel (1961), Cleopatra (1963) as Octavia, Unearthly Stranger (1964), Charlie Bubbles (1967), The Limbo Line (1968), Alfred Hitchcock's Frenzy (1972), Dark Places (1973), The Eagle Has Landed (1976), The Changeling (1980) and the fantasy films Return to Oz (1985) and Willow (1988). In 1994, she starred in a villain role in the Nickelodeon/Thames Television remake of The Tomorrow People. Her television films include Goliath Awaits (1981), See China and Die (1981), Master of the Game (1984), The Corsican Brothers (1985), A Connecticut Yankee in King Arthur's Court (1989), Fatherland (1994) for which she won a CableACE award for supporting actress, and The Pale Horse (1997). After several other guest roles in television, she played a regular supporting role on the situation comedy 9 to 5 in 1982 and 1983.
Marsh served as the presenter for International Animation Festival, an American public television series featuring award-winning animated short films from around the world. The thirteen-part series was originally broadcast in 1975 on PBS.
From 2000 until 2002, Marsh appeared in The Ghost Hunter. Her many stage credits included the West End stage revival of Boeing Boeing at the Comedy Theatre in 2007 and in Peter Hall's production of "The Portrait of a Lady" in 2008. She made an appearance in the 2008 BBC adaptation of Jane Austen's Sense and Sensibility; played the recurring character Lizzie Galbraith alongside Joanna Lumley as Davina Jackson (the lead character) in Babycow Productions' Sensitive Skin which aired on BBC Two in 2005 and 2007. She appeared in BBC Four's Crooked House in December 2008 in a role especially written for her by Mark Gatiss.
A three-part revival of Upstairs Downstairs was commissioned by the BBC with the first episode broadcast on BBC One on 26 December 2010. Marsh reprised her role as Rose Buck, who had returned to London to run an agency for domestic servants after a period spent nursing her mother in Suffolk. Eileen Atkins, who co-created the original series with Marsh, also starred in the revived series. It was set in the same London house as the original ITV series, 165 Eaton Place, resuming in 1936. Subsequently, a six-part series began transmission in February 2012 with Marsh's character appearing less frequently because of health reasons.
Marsh has also written several books: Fiennders Abbey, The House of Eliott, and Iris.
Marsh was married to future Doctor Who actor Jon Pertwee from 1955 until their divorce in 1960. She has had relationships with Albert Finney, Kenneth Haigh, and film director Michael Lindsay-Hogg.
On 3 October 2011, the BBC announced that Marsh had suffered a minor stroke and would miss the beginning of the second series of Upstairs, Downstairs. She was ultimately able to appear in only two scenes over the series, and the series was subsequently cancelled.
Marsh was appointed Officer of the Order of the British Empire (OBE) in the 2012 Birthday Honours for services to drama.The Limping Man (1953) - The Landlady's DaughterThe Love Lottery (1954) - Dancer in Sally's Dream (uncredited)The Rebel (1961) - Strange Woman at Party (uncredited)The Roman Spring of Mrs. Stone (1961) - (uncredited)Cleopatra (1963) - Octavia (uncredited)Unearthly Stranger (1964) - Miss BallardCharlie Bubbles (1967) - Waitress (uncredited)The Limbo Line (1968) - DilysA Day in the Death of Joe Egg (1972) - Woman on the Moon Rocket (uncredited)Frenzy (1972) - Monica BarlingDark Places (1973) - VictoriaThe Eagle Has Landed (1976) - Joanna GreyThe Changeling (1980) - Joanna RussellReturn to Oz (1985) - Nurse Wilson / MombiWillow (1988) - Queen BavmordaMonarch (2000) - The QueensThe Heavy (2010) - Mrs. MasonJean Marsh, The House of Eliott, Sidgwick & Jackson Ltd (Nov 1993), 978-0283061554; St Martin's Press (February 1994), ISBN 978-0-312-10996-7Jean Marsh, Fiennders Keepers, Macmillan (1996), ISBN 978-0-333-63211-6; St Martin's Press (May 1997), ISBN 978-0-312-15528-5Jean Marsh, Iris, St Martin's Press (July 2000), ISBN 978-0-312-26182-5; Macmillan (February 2003), ISBN 978-0-333-71154-5Jean Marsh, Fiennders Abbey, Pan (5 Aug 2011), ISBN 978-1-4472-0007-9