Born in Sydney, Australia, Harris was a student at the National Institute of Dramatic Art. She enrolled in 1961, aged 12, under the name Sandra Gleeson. She went on to perform in the successful Sydney stage production of Boeing Boeing (1964–65).

She had three children: Charlotte Harris (1971–2013) and Christopher (born 1972) by Dermot Harris, brother of actor Richard Harris, and son Sean Brosnan (b. 1983), by Brosnan, who adopted Charlotte and Christopher after their father died in 1986.

She appeared in The Greek Tycoon (1978), Rough Cut (1980), and the James Bond film, For Your Eyes Only (1981) as the Countess Lisl von Schlaf, the ill-fated mistress of Milos Colombo (played by Israeli actor Topol). While filming this movie, her third husband, Pierce Brosnan, met Producer Albert R. Broccoli, which eventually led to his casting as James Bond in four films. She guest starred in several episodes of the television series Remington Steele with her husband Brosnan; they married on 27 December 1980.

In 1987 Cassandra Harris was diagnosed with ovarian cancer, the same disease that had claimed her own mother's life. Cassandra Harris battled the illness for four years until her death on 28 December 1991 aged 43. Her daughter Charlotte also died from ovarian cancer on 28 June 2013 aged 41.