Sandra chose to attend the National Institute of Dramatic Art and began taking acting classes of drama in the age of 12. She enrolled to institute under the name Sandra Gleeson. To be an accomplished actress she worked very hard by attending different courses and auditions. The first acting experience came to Sandra in the 1964, when she appeared in the play “Boeing-Boeing” and worked with it till 1965. Performing with the successful play on Sydney's stage, Cassandra got a lot of experience and took a part in the some movies.

Cassandra Harris was played a few secondary roles in TV series like “Space:1999”, “Shadows” and “Enemy at the door”. Sandra began her movie actress career by playing Cassandra in the film of 1978 "The Greek Tycoon". She also appeared in "Rough Cut" in the 1980 as Mrs. Lloyd Palmer and in the film of James Bond series - “For Your Eyes Only” playing Countess Lisl von Schlaf - the ill-fated mistress of Milos Colombo. While she filming this movie, her third husband, met producer Albert R. Broccoli, which eventually led to his casting as James Bond in four movies. Appearance at the several episodes of TV series “Remington Steel” along with her husband Pierce Brosnan, was her the last work on-camera.

Cassandra married to William Firth when she was 16. They were happily married for several years and had divorced in the 1970. At the same year, in November she married to Dermot Harris after being in love affair about half a year. Dermot Harris was a Film producer from Ireland. They welcomed two children Charlotte (born in 1971) and Christopher Harris (born 1972). Sandra had divorced again in 1978. She met her third husband Pierce Brosnan before dissolved her second marriage. Obviously it was the main reason why she dissolved her second married life. Cassandra and Pierce began living together at a house in Wimbledon. After two years, on December 27, 1980 they got married. Together they had a son Sean Brosnan. Pierce also adopted Charlotte and Christopher after Dermot Harris died in the 1986. Her son Christopher became a film director, writer and producer like his own father.

She died in Los Angeles, California in 1991, she was 43 years old. At the age of 39, she was diagnosed with diagnosed of ovarian cancer. She was seriously ill for several years and died of ovarian cancer as her mother Roma in the early 60s. On June, 28 in 2013, her own daughter Charlotte died from the same disease at age 41. Her net worth is unknown.