Her father, Harold Nigel Edgerton Salmon, was a British Royal Air Force pilot killed in the war. Her mother remarried and settled in Canada, where young Joanna was adopted by her stepfather and assumed "Pettet" as her last name.

The talented, blonde Pettet studied with Sanford Meisner at the Neighborhood Playhouse, as well as at the Lincoln Center, and got her start on Broadway in such plays as Take Her, She's Mine, The Chinese Prime Minister and Poor Richard, with the late Sir Alan Bates and Gene Hackman, before she was discovered by director Sidney Lumet for his sumptuous 1966 film adaptation of Mary McCarthy's novel, The Group. Pettet made an impressive debut in that film and held her own alongside other rising young hopefuls including Candice Bergen, Shirley Knight, and Jessica Walter.

The success of that film launched a film career that included roles in Night of the Generals (1967), the James Bond spoof Casino Royale (1967), Peter Yates's Robbery with Sir Stanley Baker (1967), the strange Western drama Blue (1968) with Terence Stamp, and the Victorian period comedy The Best House in London (1969). In 1968 she married American actor Alex Cord and gave birth to a son later that year. She and Cord were divorced in 1989 after 21 years of marriage and she never remarried.

Although she co-starred with actor Rod Taylor in the 1980 thriller, Cry of the Innocent, her feature film appearances became sporadic. However, Pettet re-emerged as the star of over a dozen made-for-television movies, including The Delphi Bureau (1972), The Weekend Nun (1972), Footsteps (1972), Pioneer Woman (1973), A Cry in the Wilderness (1974), The Desperate Miles (1975), The Hancocks (1976), Sex and the Married Woman (1977), and The Return of Frank Cannon (1980). She also starred in the NBC miniseries Captains and the Kings (1976), guest-starred four times on the classic Rod Serling anthology series Night Gallery, was a frequent guest on both Fantasy Island and The Love Boat (appearing three separate times on each series), and had a recurring role on Knots Landing in 1983 as an LAPD homicide detective investigating the murder of singer Ciji Dunne (played by Lisa Hartman).

Pettet also enjoyed some success as an unofficial "scream queen" with appearances in such horror films as Welcome to Arrow Beach (1974), The Evil (1978), and the slasher-mystery Double Exposure (1982), as well as having made two appearances on the Brian Clemens produced Thriller television series in the UK. Her most notable film role in the 1980s was in Michael Cacoyannis's political drama Sweet Country (1986), which dramatized the turmoil in Chile following the 1973 assassination of Marxist President Salvador Allende, featuring Jane Alexander, Franco Nero, and Irene Papas.

Pettet's career slowed down in the late-1980s and her last acting appearance was in a "bad action film" called Terror in Paradise in 1990 that was produced by Roger Corman and his frequent Philippine associate Cirio Santiago. During filming in the Philippines she was held hostage by rebels, led by Gregorio Honasan, attempting to overthrow Corazon Aquino, and managed to escape the hotel where she was being held before getting out of the country. By then, she had already lost her enthusiasm for acting and decided that it was time to bow out gracefully from the entertainment industry.

The sudden and unexpected death in 1995 of her only child, son Damien Zachary Cord, at the age of 26 was a crushing tragedy that nearly destroyed Pettet. The shock and grief she experienced over his death caused her to retreat even further from Hollywood. For a period of time, she lived a reclusive existence in the California desert until she eventually moved to London.

Joanna Pettet had been a close friend of actress Sharon Tate and visited Tate for several hours at her Benedict Canyon home on the afternoon of August 8, 1969--the day Tate was murdered by the Manson Family. (She was also a close friend of both Janice Wylie and Emily Hoffert, the murder victims from the infamous "Career Girl Murders" in New York during the 1960s that became the basis of the 1973 TV movie The Marcus-Nelson Murders and launched the Kojak television series).

More recently, she was the final companion and caregiver to the esteemed British actor, Sir Alan Bates, who died of pancreatic cancer in London, England in 2003, aged 69. The two actors had been lifelong friends for nearly 40 years.