When No Blade of Grass (1970) was released, the film received mixed reviews from critics. Despite the lukewarm reception of the film, it made Frederick an overnight sensation, and her career quickly skyrocketed. Represented by the prestigious talent agency, Hazel Malone Management, Frederick became a teen idol among the British public in the early 1970s, achieving the success and popularity equivalent to that of Hayley Mills and Olivia Hussey. She was regularly featured in newspaper articles and fashion magazines as a model and cover girl. Her most notable spread was in the British Vogue in September 1971, where she was photographed by Patrick Lichfield. In addition, she also appeared in several television commercials for products that include Camay soap. Frederick then signed a cosmetics contract with Mennen, and became a spokesmodel for Protein 21 shampoo, starring in nationwide campaign print and television ads. A British national newspaper chose her as its “Face of 1971”, and she was hailed as one of Hollywood’s most promising newcomers.

In 1971 she appeared in the biographical film Nicholas and Alexandra (1971), in which she played the Grand Duchess Tatiana Nikolaevna of Russia, second eldest daughter of Tsar Nicholas II. For the film’s press tour, she toured Europe with her three co-stars Ania Marson, Candace Glendenning, and Fiona Fullerton. That same year, she auditioned for the role of Alice in Alice’s Adventures in Wonderland (1972), but lost the role to her friend and Nicholas and Alexandra co-star, Fiona Fullerton. She was also first runner-up for the role of Saint Clare of Assisi in the Franco Zeffirelli production of Brother Sun, Sister Moon (1972), which ultimately went to Judi Bowker.

Her best-known appearance was in 1972 where she played Catherine Howard, in Henry VIII and His Six Wives. Her next role was in the 1972 family film The Amazing Mr. Blunden; in 1973, she won the Evening Standard British Film Award for Best New Actress. She continued to work in various film and television projects throughout 1973 and 1974 where she often was cast in the archetype of the girl next door, an ingénue, or a princess. Some of the shows she appeared on were Follyfoot, The Generation Game, and an adaptation of The Canterville Ghost where she first met David Niven, who became a lifelong friend.

One of Frederick’s closest friends was Mauritian actress, Françoise Pascal. The two first met when they co-starred on a 1972 episode of the television anthology series, BBC Play of the Month, and quickly became “firm friends”. Pascal recalled that they remained friends for several years before regretfully losing touch after Frederick married Sellers in 1977. In April 2020, a few weeks before the 26th anniversary of Frederick’s death, Pascal tweeted a photo of herself and Frederick, with the caption “I think of her very often! Always had that fresh baby face! RIP Lynne! Xxx”.

Frederick’s most prominent television role came in 1974 where she appeared on three episodes of the critically acclaimed and Emmy-winning series The Pallisers. The series featured a huge cast of prominent and rising British actors, including Anthony Andrews, of whom she played the love interest.

In a 1975 interview with Men Only, Frederick discussed that she “partially agreed” with Women’s Lib. Adding “I agree with the fact that women should have equal rights”, but adding that she also believed in some old fashioned gender roles. “I agree that there are certain things that men are designed to do; just as there are things women are designed to do.”

Frederick began 1976 with an appearance on a controversial episode of the BBC series Play for Today, titled “The Other Woman”, in which she played a sexually enigmatic girl who falls for a lesbian artist played by Jane Lapotaire. Later the same year, she delivered a critically acclaimed performance in the Oscar-nominated film, Voyage of the Damned (1976). She followed that with a leading role in a Pete Walker slasher horror film, Schizo (1976), a movie that became an underground hit in the horror film community.

Frederick’s first marriage at age 22 was to Peter Sellers. They met at a Dennis Selinger dinner party in 1976 after Frederick had finished making Schizo (1976). Sellers first proposed to her two days after their first meeting, but she turned him down. They dated for a year before he proposed to her again. They eloped in Paris on February 18, 1977.

Following her marriage to Peter Sellers, Frederick’s career stalled for over a year as Sellers forced her to turn down all the acting offers she was receiving, to tend him through poor health. This included supporting and looking after him on the sets of his films. She attempted to make a career comeback in 1978, but the year long absence had cost Frederick her burgeoning stardom.

Frederick campaigned and auditioned for several films. The role that she most desired, and spent a great deal of time lobbying herself for, was the leading role of Meggie Cleary in The Thorn Birds. Despite her lengthy and accomplished acting résumé, the producers decided they wanted a much bigger catch. Other roles she campaigned for included Cosette in the 1978 television adaptation of Les Misérables (1978), and Anne Sullivan in the television remake of The Miracle Worker (1979), none of which she received. She made her final onscreen appearance with her husband, Sellers, in the 1979 remake of The Prisoner of Zenda, which was a box-office and critical flop. Her final credit was as an executive producer on Sellers’ last film The Fiendish Plot of Dr. Fu Manchu.

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 finalised, she inherited almost his entire estate, worth an estimated £4.5 million (£19.4 million today), and his children received £800 each (£3,456 today). Despite appeals from a number of Sellers’ friends to make a fair settlement to the children, Frederick refused to give her stepchildren anything due to their rocky relationship with her and Peter. After Sellers’ death, her stepson, Michael Sellers, published an exposé memoir on his relationship with his father, P.S. I Love You: An Intimate Portrait of Peter Sellers. In the book he accused Frederick of being a deceitful, cunning, and narcissistic fraud who only married his father for his money. He also made allegations that Frederick had cheated his sisters and him out of their inheritance by intentionally manipulating their father to alter the will in her favour. This led to the press vilifying and labeling her as a “gold digger”.

After Sellers’ death in 1980, UA, wanting to cash in on the continuation of the series, elected Edwards to construct a new film from outtakes and deleted scenes from the five previous Pink Panther movies featuring Sellers. A handful of new material involving other actors was filmed for this movie. Some of the older material dated as far back as nineteen years before this movie was made, to 1963. The negligence in continuity was evident in many scenes, and was subjected to heavy mockery from film critics.

She briefly married David Frost (on 25 January 1981), and her supposed eagerness to remarry so quickly after Sellers’ death caused a loss of dignity in the public eye, and was one of the major factors in her blacklisting. Frederick divorced Frost after 17 months. During the course of their marriage, she suffered a miscarriage in March 1982.

In December 1982, she married California surgeon and heart specialist Dr. Barry Unger with whom she bore her only child, Cassie Cecilia Unger (born 1983); they divorced in 1991.

In 1982, Frederick’s screen appearance as Catherine Howard from the film Henry VIII and His Six Wives (1972), was used on the cover art for the 1982 novel The Dark Rose by Cynthia Harrod-Eagles.

Frederick, who never met her biological father, regarded actor David Niven as her adopted father figure. They first met while filming the television film adaptation of The Canterville Ghost (1974). They remained close friends over the years until Niven’s death in 1983, which occurred just eight weeks after the birth of her daughter. As a child, she was very close with her mother, Iris, and grandmother, Cecilia, but was estranged from both of them during the course of her marriage to Sellers. Iris said of her daughter’s marriage: “How could my daughter marry someone like Peter Sellers with his track record? The marriage is doomed from the start. My own marriage ended unhappily when Lynne was two. I tried to compensate for her having no father by devoting all the time I wasn’t working to her. Perhaps if I had married again she wouldn’t have gone on choosing men twice her age as boyfriends – looking for a father figure I suppose“.

In 1985 Judge Charles Hobhouse ruled in favour of Frederick, awarding her $1 million dollars, but dismissed her request to ban the film. Despite her noble intentions to protect Sellers’ legacy, the press further sneered at her. Frederick herself stated “I hope this proves that I’m not a gold digger! I’ve risked my entire fortune and the financial future of my daughter to protect Peter’s reputation.” After the lawsuit, Frederick continued to guard Sellers’ films and went to great lengths to make sure each one was handled with respect and dignity.

Frederick’s relationship with her stepchildren (Michael Sellers, Sarah Sellers, and Victoria Sellers) was, like Peter’s relationship with them, distant and often strained. When Lynne began her relationship with Peter, she made efforts to establish a friendly connection with them. Sarah recalled of Lynne, “she seemed quite nice to begin with. I actually told dad that I thought she was a bit stupid. But she came across as very bubbly, friendly, warm and interested. But once they got married things definitely changed”. Michael Sellers shared his thoughts of Frederick in his exposé memoir where he said “my first impression of Lynne didn’t do much to alter my views. She was not exactly my idea of sweetness and light. It didn’t concern me that she lacked the good looks of dad’s past wives and girlfriends, but those innocent eyes, certainly her strongest feature, didn’t deceive me”. Michael Sellers also bluntly acknowledged his intentional hostility and lack of respect towards Lynne when they first met: “I’m afraid we weren’t very kind in our judgement of Lynne. Sarah thought she wasn’t too bright. But our views didn’t really count for much. Because whatever our opinions, they would be of purely academic interest”. Months after Frederick’s death in 1994, Victoria remarked “I feel now that she’s in hell – I don’t know but that makes me feel better.”

On 27 April 1994, Frederick was found dead by her mother in her West Los Angeles home, aged 39. Foul play and suicide were ruled out and an autopsy failed to determine the cause of death, and some in the media speculated she died from the effects of alcoholism. Her remains were cremated at Golders Green Crematorium in London, and her ashes were interred with those of her first husband, Peter Sellers.

In a 1995 interview with Hello! magazine, her mother, Iris, claimed that Lynne died from natural causes which was brought on by a seizure in her sleep. She also denied accusations that her daughter had a drug or alcohol problem.

Over time, views towards her image gradually shifted, and she soon gained a cult following through her films, and has been described as one of the most promising, talented, beautiful, and ascending young British actresses of the 1970s. Many credit the negative events in her life (the loss of her acting career, blacklisting in Hollywood, and untimely death) to her marriage to Sellers. Even Roger Lewis, who was blunt about his disdain for Frederick, admitted that “of all of Sellers’s wives, Lynne Frederick was the most poorly treated”. One of the first people to advocate for Lynne was American author, Ed Sikov, in the 2002 book, Mr. Strangelove: A Biography of Peter Sellers: “Lynne Frederick deserves a bit of compassion herself in retrospect. It was the helpless Peter she nursed, the dependent and infantile creature of impulse and consequent contradiction. Patiently she ministered him”. Other people who have defended or come forward with positive recollections of Frederick over the years include Judy Matheson, Françoise Pascal, John Moulder-Brown, Mark Burns, Fabio Testi, Ty Jeffries, Lionel Jeffries, David Niven, and Graham Crowden.

In the years after her death, Frederick’s legacy remained poisoned and she seldom was talked about in favourable terms. In the 2004 book The Life and Death of Peter Sellers, Roger Lewis claimed that “there is yet to find a single person to say a good thing about Lynne”. British journalist Nigel Dempster had a profound dislike for Frederick and referred to her as an “avaricious and cunning man-eater”. Other people who have voiced unfavorable views of Lynne include Spike Milligan, Britt Ekland, Sir Roger Moore, Wendy Richard and Simon Williams.

She received minimal attention in the 2004 movie adaptation of Lewis’s book where she was portrayed by British actress Emilia Fox. All scenes featuring Fox’s portrayal of Frederick were deleted from the final cut of the film, but included in the supplemental features of the film’s DVD release. On portraying Frederick, Fox stated “I had thought very carefully about playing Lynne. I wanted to represent her in a way that I thought was fair – which was a very young girl being taken up in this world of laughter and light, and then finding out the reality. Peter Sellers was completely obsessed by work and it’s very difficult to live with someone like that.”

In her 2014 memoir I Said Yes to Everything, Lee Grant claimed that during production of the film Voyage of the Damned (1976) Frederick, then aged 21, engaged in an affair with Sam Wanamaker, who was 35 years Frederick’s senior and married to Charlotte Holland at the time. Grant also stated that she witnessed all the men on set, including the film’s director Stuart Rosenberg, make salacious passes at Frederick, all of which she rejected.

In 2018, Judy Matheson revealed that she had worked with Frederick in the early 1970s. They were slated to appear in a film together that was to be shot in the Netherlands, with John Hamil, Robert Coleby, and Nina Francis. Because Frederick was young and a relative newcomer to filmmaking at the time, Matheson (who was a few years older and had industry experience) was asked to be Lynne’s chaperone for the trip (as Lynne’s mother was unavailable). They spent about three weeks lodged together in a hotel room before production on the film was prematurely closed due to financial withdrawals. Matheson stated that she enjoyed Frederick’s company, and that they managed to have fun together despite the production struggles. After returning to Great Britain, they corresponded for a while before gradually losing touch with each other.

Julie Andrews stated in her 2019 autobiography Home Work: A Memoir of My Hollywood Years that she suspected her husband Blake Edwards was having an affair with Lynne (who was married to Sellers at the time) during production of Revenge of the Pink Panther (1978). When Andrews confronted Blake about the “flirtations” between him and Frederick, Julie asked him point-blank which he preferred: staying married or continuing this flirtation. After this confrontation, Blake ceased all alleged flirtations with Frederick. She later had a fall-out with Edwards and Andrews after successfully suing them for their involvement with the film Trail of the Pink Panther (1982), claiming that it insulted Sellers’ memory. She never spoke to them again.