Brought up at "Carnlea" overlooking Calderstones Park in Liverpool and later, 18 The Ridge at Purley in Surrey, Sim was privately educated before training at RADA.

She appeared in the Powell and Pressburger film, A Canterbury Tale (1944); she acted alongside her husband in the Boulting brothers' The Guinea Pig (1948); and starred opposite Anthony Steel in West of Zanzibar (1954).

Sim married Richard Attenborough on 22 January 1945 and they had lived in a house on Richmond Green in London from 1956 until 2012, when her husband placed it for sale at £11.5 million.

[3] In June 2012, shortly before her 90th birthday, Sim entered the actors' retirement home Denville Hall, for which she and her husband had helped raise funds.

[7][8] She was cremated and her ashes were interred in a vault at St Mary Magdalene church in Richmond beside those of her husband, as well as her daughter Jane Holland and her granddaughter, Lucy, who died in the 2004 Boxing Day tsunami.