Producer Val Guest lifted Simmons out of a dancing school in the dreary suburb of Cricklewood after she had attended just two weeks tuition. Guest saw in Simmons the fresh face needed for his upcoming film Give Us the Moon (1943). Simmons was fourteen and her father warned her luck would not last. He was wrong.

Simmons' beauty and genuine, if untrained, talent carried her from success to success. In 1946, she played Estella in Great Expectations alongside soon to be famous actors John Mills and Alec Guinness. Two years later, Laurence Olivier desperate that she play Ophelia in his new production of Hamlet, pleaded with Rank organisation (to whom she was contracted) for 30 days of her time. They agreed.

Hamlet catapulted Simmons to international stardom; she was nominated for two Oscars and her face covered Time magazine. Next she went to Fiji to make The Blue Lagoon. Married to swashbuckling actor Stewart Granger in 1950, the couple moved to Hollywood and had a daughter six years later. The union lasted a decade. Ignoring the fact Simmons was married; Howard Hughes wanted her. He bought her contract and then tried to force her to sign a new one when it expired. Simmons refused. Hughes tried to wreck her career, threatening to sue other studios should they hired her. The matter was finally settled out of court.

Simmons married the writer-director Richard Brook in 1960, staying off screen for a few years until Spartacus (1963).

She continued to work through out the 70's, 80's and 90's in film and television. In the mid 80s she sought professional help after becoming addicted to alcohol due to being depressed about the poor roles she was being offered.

She lived in Santa Monica, California for many years.