Cynthia Powell was born in Blackpool, England on September 10, 1939 to Charles and Lilian Powell. Cynthia had two older brothers, Charles and Anthony. Her father, Charles, was a commercial salesman, and would accompany Cynthia by bus on their way to school and work. Her mother, Lilian, loved auction sales and would often rearrange and redecorate the family furniture.  However, tragedy struck the family when Cynthia was 17 years old in June of 1957, her father died of lung cancer. Cynthia was devastated but Lilian encouraged her to continue on with her studies in art, enrolling in the Liverpool College of Art in Liverpool, England.

Cynthia met John Lennon, another student at Liverpool College of Art, who she has seen in the hallways and in Lettering class. Cynthia wasn't at all impressed- John would borrow her belongings without returning them, called her "Miss Prim" or "Miss Powell", and was dating Thelma Pickles while Cynthia was dating Barry, who they were thinking about getting engaged. It wasn't until one day in class when a friend of both John and Cynthia's, Helen Anderson, playfully reached over to John's hair to straighten it out from a hairsyle she didn't like. Cynthia was disturbed by the sight and realized that she was jealous! Cynthia became fascinated by John, coming into class earlier and leaving school later while John would constantly bring his guitar everywhere and played ballads to Cynthia, particularly "Ain't She Sweet". At the end of the school year, there was a party to which Cynthia and her childhood best friend, Phyllis Mackenzie, attended and to Cynthia's surprised, John was there as well with his friends. He asked Cynthia to dance and when the dance ended, John asked Cynthia to see him but she told him that she was engaged. John walked off while Cynthia realized it was a dumb move and started to leave but John stopped her and asked for her to stay. By the end of the day, John and Cynthia were together.

After four years together, in the summer of 1962, Cynthia discovered that she was pregnant and told John. They got married on August 23, 1962 in Liverpool, England with Cynthia's brother Tony and his wife Marjorie, Paul McCartney, George Harrison, and Brian Epstein, the Beatles' manager, serving as best man. At Brian's request, Cynthia and the pregnancy/birth was to be kept secret so fans would believe that John was single so it wouldn't ruin the popularity, to which John was against but went on with it for a while. On April 8, 1963, Cynthia gave birth to John Charles Julian Lennon in Liverpool, England while John was on tour; it took three days until John came to the hospital for a visit to meet his son for the first time. In the fall, John and Cynthia took their belated honeymoon in Paris, France while John's Aunt Harriet Stanley Birch and his two sisters, Julia and Jacqui, looked after Julian. By late 1963-early 1964, John's marriage and fatherhood was out, known to the public, and John decided that Cynthia could accompany him to The Beatles' first United States trip. By 1964, John, Cynthia, and Julian moved to Weybridge in Surrey, England for the next four years until the divorce in 1968. During the pressures and stress of touring, recording, writing, filming TV/Film appearances for The Beatles' success, John turned to marijuana and LSD; Cynthia didn't mind the marijuana but had a few horrible experiences with LSD refusing to take it which caused a rift in the marriage. Although John was unfaithful during the marriage, it wasn't until he fell in love with Yoko Ono, whom he met in 1966, that he decided to leave Cynthia. After the divorce was final, Cynthia and Julian moved many times near Liverpool and Wales over the years and led a private life with the exception of a few interviews. Cynthia's relationship with John was strained despite a few attempts to communicate in regards of their son; Julian's relationship with John was also strained but during the last few years of John's life, they were getting close. John was murdered on December 8, 1980 in New York City, New York with his wife, Yoko Ono at his side. Cynthia and Julian were devastated, and Julian traveled to New York to be with Yoko and his younger brother Sean for a few months.

After her divorce from John, Cynthia remarried Roberto Bassanini, an Italian restaurateur who's family ran a hotel business in Italy where she first met him in 1968 while on holiday with Julian and her mother (also where she found out that John wanted to divorce her and be with Yoko Ono) on July 31, 1970 with Julian as a page boy. They were married for three years until their divorce. Cynthia remarried the third time to John Twist, an engineer, on May 1, 1976. Together they ran Oliver's Twist, a hotel-and-restaurant in Wales with Angie McCartney, ex-sister in law of Paul McCartney. In 1978, Cynthia released her first autobiography, A Twist of Lennon. By 1983, the marriage was over and Cynthia moved to Isle of Man with boyfriend Jim Christie. Cynthia started many projects as a television personality, creating her own perfume "Woman", opening a few restaurants with Bunter's and Lennon's, releasing a single "Those Were the Days", auctioning her belongings from her time with John Lennon, and going into Interior design business. In 1999, Cynthia and Jim broke up; by the following year, her son Julian introduced Cynthia to his long time friend, Noel Charles, a nightclub owner. Cynthia and Noel got married on June 7, 2002 and live in Spain. In 2005, Cynthia released her second autobiography, John. Cynthia remains close to her son, Julian. On March 10, 2013, Noel passed away in his sleep from battling cancer after almost two years. Noel and Cynthia were married for almost 11 years.

In early 2015, Cynthia discovered that she had cancer and it was already too late; on April 1, 2015, Cynthia died in her home with her son Julian at her side.