Frederick was born in Hillingdon, Middlesex to Andrew Frederick (1914-1983) and Iris C. (née Sullivan) Frederick (1928-2006). Her mother became a casting director for Thames Television. Lynne’s parents separated when she was two years old, and she was brought up by her mother, Iris, and her grandmother, Cecilia, at Market Harborough, Leicestershire.  Having originally aspired to becoming a teacher of mathematics and physics, she abandoned her academic pursuits for the stage, and made her film debut as Mary Custance in No Blade of Grass (1970), when she was 16 years old. She appeared a year later in the 1971 biographical film Nicholas and Alexandra, in which she played Grand Duchess Tatiana Nikolaevna of Russia, second eldest daughter of Tsar Nicholas II. However her best-known appearance came shortly afterwards when she played another historical character, Catherine Howard in Henry VIII and His Six Wives in 1972. Frederick would go on to pursue a successful career in films throughout the 1970s. Her next role was in the 1972 children’s film The Amazing Mr. Blunden and in 1973 she won the Evening Standard British Film Award for Best New Actress. Other notable films included Saul Bass’ science fiction thriller Phase IV (1974), the Spanish romance A Long Return (1975), and Schizo (1976). Her last role came in the 1979 film The Prisoner of Zenda, in which she worked with her first husband Peter Sellers.  In 1985, she was offered the role of Kumiko in The Karate Kid, Part II. At the time the script was written in mind for an English actress who was to play a half English, half-Japanese village girl adopted by her Japanese aunt. Frederick, who hadn’t appeared in a theatrical release since The Prisoner of Zenda in 1979, had been planning an acting comeback for quite some time. Despite her interest in the script, she turned the offer down, preferring instead to concentrate on motherhood as she had given birth to a daughter the previous year.

Frederick’s first marriage, at age 22, was to Peter Sellers on 18 February 1977. The marriage was rocky, and Sellers was reportedly in the process of excluding her from his will a week before he died of a heart attack on 24 July 1980, the day before her 26th birthday. The planned changes to the will not having been finalized, she inherited almost his entire estate worth an estimated £4.5 million (£17.2 million today) while his children, whom he had disinherited, received £800 each (£3,064 today). Despite appeals from a number of Sellers’ friends to make a fairer settlement to the children, Frederick refused to give her stepchildren anything. She later won nearly £1 million (£3.2 million today) in a lawsuit against the makers of the Trail of the Pink Panther (1982), made after Sellers’s death, claiming the film tarnished her late husband’s memory.[4] She reportedly suffered from severe depression because of Sellers’s death and attempted suicide numerous times. Over her remaining 14 years following Sellers’ death, she reportedly became obsessed by his memory and kept a shrine to him at their Swiss chalet in Gstaad, which she inherited from him.  She briefly married David Frost (on 25 January 1981); they divorced 17 months later. She later married a Californian surgeon and heart specialist, Dr. Barry Unger, in December 1982; they were divorced in 1991. In her last marriage, she bore her only child, Cassie Unger (b. 1983).

In her later years, her health deteriorated. On 27 April 1994 she was found dead in her West Los Angeles home, aged 39. Victoria, Peter Sellers’s daughter by his previous marriage to Britt Ekland, last saw Lynne three weeks before her death and subsequently stated, “I was so shocked. Lynne was sitting in her kitchen, dressed in a filthy kaftan. She could hardly move. She was swigging vodka directly from a jug with a handle on the side”.  There was no evidence of foul play, and although suicide was suspected by some, a post-mortem failed to determine the cause of death. She was survived by her mother, Iris, and her daughter, Cassie. Her remains were cremated at Golders Green Crematorium in London and her ashes were mingled and then interred with those of Peter Sellers. Her estate passed into a trust fund, the Lynne Unger Children’s Trust, which was administered by accountants in Santa Monica, California; her daughter was the beneficiary.