Born Pamela Helen Ostrer in Southend-on-Sea, Essex, Mason was the daughter of Isidore (1889-1975) and Helen Ostrer. Isidore Ostrer was a wealthy industrialist and banker who became president of the Gaumont British Picture Corporation in the early 1920s. Pamela left school at age 9, and married cinematographer Roy Kellino at age 16 in 1932. In 1935, Pamela met actor James Mason on the set of his second film, Troubled Waters, on which her husband Roy was working as a cinematographer. Mason and Pamela were quickly attracted to each other. Mason became close friends with both Kellinos, moved in with them, and collaborated with them on several stage and screen projects, culminating in the 1938 film I Met a Murderer, in which Mason and Pamela played lovers on the run. Shortly afterwards, Roy Kellino divorced Pamela naming Mason as correspondent, and she married Mason in 1940. Kellino remained on friendly terms with the Masons and directed their later films Lady Possessed and Charade. After her divorce and remarriage, Pamela continued to use the name “Pamela Kellino” for some years in her acting and writing work. The Masons relocated from London to Hollywood in the late 1940s, occupying the mansion previously owned by Buster Keaton, where Pamela became a popular hostess of frequent parties. They had two children: daughter Portland (1948–2004), and son Morgan (who later became an advisor to President Ronald Reagan and married Belinda Carlisle, the lead singer of the Go-Go’s). Portland Mason was named for the Masons’ friend Portland Hoffa, the wife of the American radio comedian Fred Allen. Pamela filed suit for divorce from James Mason in 1962, claiming that he had committed adultery. According to the Masons’ son Morgan and other sources, Pamela herself had had numerous affairs, but due to the skill of her attorney Marvin Mitchelson, when the marriage was finally dissolved in 1964 she still won a large monetary settlement of at least $1 million, reported as “America’s first million-dollar divorce”. As a result of his success on Pamela’s behalf, Mitchelson became a sought-after celebrity divorce attorney. Following her divorce, Pamela continued to live in the Keaton mansion in Beverly Hills until her death.

Pamela (as Pamela Kellino) made her film debut in 1934 in the Gaumont British big-budget film Jew Süss. She remained under contract to Gaumont British (her father’s film company) for several years, despite acting in films only sporadically while also working as a screenwriter, producer, and author. From the late 1930s through the 1950s, Pamela Mason (often credited as Pamela Kellino, including after her marriage to James Mason), wrote, produced and/or appeared in several films in collaboration with James Mason and/or Roy Kellino. Most notably, she co-starred with James Mason in the films The Upturned Glass and Charade (directed by Roy Kellino), both of which she also co-wrote. The Masons co-produced the films I Met a Murderer and Lady Possessed, both of which were directed by Roy Kellino and lost money. Pamela also had small roles in a number of other films starring James Mason. Later films in which she appeared without James Mason included The Child (1954) (a short film directed by James, in which daughter Portland also appeared), Sex Kittens Go to College (1960) and The Sandpiper (1965).

In the mid-1950s, Pamela Mason appeared with her husband James Mason on a short-lived variety show, The James Mason Show. She was a contestant on the February 18, 1960 episode of the TV quiz show You Bet Your Life, hosted by Groucho Marx. In the 1960s, she hosted two talk shows: The Pamela Mason Show from 1965–1966, and The Weaker (?) Sex, which featured female guests, from 1968-1969. From the late 1950s through the 1970s, she made occasional appearances as a guest star on various TV series, including Playhouse 90, Love, American Style, and Wonder Woman. Her last acting appearance was in a made-for-television biopic of Errol Flynn in 1985. In addition to her screenwriting work, Pamela Mason authored a number of books, both fiction and non-fiction, some of which were published under the name “Pamela Kellino”. Her novel Del Palma (1948), dismissed by Kirkus Reviews as “trash”, became the basis for the film Lady Possessed, which the Masons co-produced. Other titles by Mason include the novel Ignoramus, Ignoramus (1950) (illustrated by James Mason); The Cats In Our Lives (1949), about the cats and other animals owned by the Masons (co-written and illustrated by James); Marriage Is the First Step Toward Divorce (1968); and The Female Pleasure Hunt (1972). On 29 June 1996, Mason died of heart failure at her home in Beverly Hills, California. She was survived by her daughter and her son. She is buried in Westwood Village Memorial Park Cemetery in Los Angeles.