With a career spanning seven decades, Johns is often cited as the "complete actress". Johns is a trained pianist and singer. She is also an accomplished dancer, and was qualified to teach ballet at the age of ten.

Johns is the daughter of the Welsh actor, Mervyn Johns (1899 - 1992), and her roots are in West Wales, although she was born in Pretoria, South Africa, while her parents were on tour there. Her mother, Alys Steele, was a concert pianist and her only child, her son, Gareth Forwood, is a British actor.

Johns made her film debut in 1938, in the movie version of Winifred Holtby's novel, South Riding. In 1944 she appeared opposite her father in Halfway House, and in 1948 starred as a mermaid in Miranda. She successfully made the transition to Hollywood, appearing in The Court Jester (1956) as Danny Kaye's love interest.

One of her best known film roles was that of Winifred Banks, the children's mother, a suffragette, in Mary Poppins (1964).

Johns has also appeared on television and on stage, most memorably in Stephen Sondheim's musical, A Little Night Music. The song, Send in the Clowns, was written with her in mind, and in 1973 she won a Tony award for her role in the musical. She had a brief television series in the U.S. called Glynis.

Johns has been married four times, including a marriage to Anthony Forwood, father of her only child, her son Gareth, and the former manager and long-time partner of Sir Dirk Bogarde. She once remarked that she was wed so often because she married all her lovers; Dame Elizabeth Taylor made a similar statement about her own many marriages. Glynis also married David Foster, who at the time, was chairman of Colgate Palmolive International.

Johns has been the subject of countless magazine covers and pin-up posters throughout her long career, which appears to have ended after many decades of hard work on stage, in films and on TV.