Born Joan Mildred Summerfield in Brixton, London, the only child of variety performers Norman Field (né Summerfield) and Nina Norre, she started her theatrical career in 1931 as a Dancer.

She signed to Gainsborough Pictures during the Second World War. Kent had small roles in It's That Man Again (1943), Miss London Ltd. (1943) and Warn That Man (1944).

The turning point in her career came when she was given a dramatic part in the Gainsborough melodrama film Fanny by Gaslight (1944). She played a part turned down by Margaret Lockwood – the childhood friend of Phyllis Calvert who becomes the mistress of James Mason. The movie, also starring Stewart Granger, was popular in England and established Kent as Gainsborough's back up to Margaret Lockwood.

Kent played another sexually aggressive girl in Madonna of the Seven Moons (1945) with Calvert and Granger. It was a big hit. Rank borrowed her to support Rex Harrison in The Rake's Progress (1945) then back at Gainsborough she was in Waterloo Road (1945) with John Mills and Granger.

Kent was married to Austrian actor Josef Ramart from 1946 until his death in 1989, aged 70. They met on the set of Caravan. Actor Stewart Granger was the best man at their wedding. They appeared together in the films Caravan and Trottie True. She was the subject of This Is Your Life in 1974 when she was surprised by Eamonn Andrews at the Strand Theatre. Kent made her last public appearance in June 2011, when she was honoured by the British Film Institute on her 90th birthday.

Kent was top billed as one of several names in Bond Street (1948) and was the female lead in a thriller 'Sleeping Car to Trieste (1948), playing a spy.

Kent had her best chance yet playing the lead in a musical Trottie True (1949) which became her favorite film. She made a comedy in Italy, Her Favourite Husband (1950) and appeared opposite Dirk Bogarde in The Woman in Question (1950).

Kent's film appearances grew less frequent from the mid 1950s onward. She had support roles in The Prince and the Showgirl (1957) and Bonjour Tristesse (1958) and a good part in the horror film The Haunted Strangler (1959). She was in the comedy Please Turn Over (1959) and the thriller Beyond This Place (1959). She was one of several female stars in Bluebeard's Ten Honeymoons (1960) with George Sanders.

She starred in the melodrama The Reluctant Widow (1951) then had a good role as the wife in The Browning Version (1951).

Kent was in a thriller The Lost Hours (1952) with American actor Mark Stevens and Before I Wake (1954). In 1953 she was in a play Uncertain Joy.

In 1954 she fell ill while touring in a stage production of The Deep Blue Sea in South Africa.

She played Queen Elizabeth I in the historical TV adventure series Sir Francis Drake filmed in 1961–62.

Kent died in the West Suffolk Hospital, Bury St. Edmunds on 30 November 2013, following a fall at her home in Westhorpe. The coroner recorded a narrative verdict that Kent died from accidental injuries and that cardiac disease may have contributed to a fall.