Judith Campbell Exner, 65, a suburban California woman who made headlines in the mid-1970s when her named was linked to President John F. Kennedy as well as the nation's reputedly most powerful organized crime boss, died Sept. 24 in Duarte, Calif.

Ms. Exner, who lived in Newport Beach, Calif., died at a hospital of breast cancer.

In 1975, her name and accounts of her romantic involvements began leaking from Senate investigations of alleged CIA assassination attempts of foreign leaders. Her name came to light after it was alleged that Kennedy had tried to make use of Mafia figures in an effort to get rid of Cuban communist leader Fidel Castro. It was reported that she was the girlfriend of one of the most prominent of Mafia chieftains, Sam Giancana.

Reports at the time alleged that the link between Giancana and the White House was a mystery woman who was at one time or another sexually involved with the president and the gangster.

Ms. Exner, who first denied acting as a go-between, claimed she did have affairs with both men, though not at the same time.

This claim was asserted in a book that appeared under her name; it said she met and began her relationship with Kennedy in February 1960, when the Democrat was still the junior senator from Massachusetts, and ended it in 1962 while he was president.

She said she met Giancana later, in a nightclub scene featuring singer Frank Sinatra. She said she believed the notorious and highly publicized crime lord was a "Chicago businessman" when she began her affair and continued the affair after she learned otherwise. Giancana was killed in 1975.

Judith Eileen Katherine Immoor Campbell Exner was born in New York and grew up in Los Angeles. She was born the fourth of five children to a father who was an architect and real estate investor. The family counted comedian Bob Hope and his wife among its neighbors and friends.

At the age of 18, Ms. Exner married William Campbell, a television actor, whom she divorced in 1958. Through her first marriage, she met numerous show business figures, including Sinatra.

It was Sinatra who invited her to the Las Vegas show in which he starred along with Dean Martin, Sammy Davis Jr. and Peter Lawford, which Kennedy attended. According to Ms. Exner's lawyer, James H. Lesar, she and Kennedy began their relationship in Las Vegas. Their relationship blossomed at meetings at New York hotels, Kennedy's Georgetown home, and finally, 1600 Pennsylvania Ave.

She said in a Washington Post account that she lost some of her regard for Kennedy when he showed up at one of their meetings with another woman and suggested a "threesome." Although the relationship ended at the end of 1962, she said, they remained in telephone contact until shortly before the president's death.

When Ms. Exner was questioned by Senate investigators, three months after Giancana was shot to death in his home, she told the investigators she feared for her life if her name or story leaked to the public.

After her story leaked, and such Kennedy loyalists as Evelyn Lincoln, Dave Powers and Kenneth O'Donnell publicly denied her story, she hit on a different strategy.

In 1977, her book, "My Story," hit the bookstores. Reviewing it for The Post, William B. Furlong said the book included "some of the most banal prose and dreadful dialogue in contemporary literature. What is surprising is a most improbable feat of portraiture: Jack Kennedy emerges as a bore."

It was reported that another trenchant literary critic, Frank Sinatra, commenting on the book, remarked that "Hell hath no fury like a hustler with a literary agent."

Then, in 1988, Ms. Exner changed her story. She told People magazine that her contacts with Giancana and another mob figure, Johnny Rosselli, were on behalf of President Kennedy. She later maintained that she had transported money and documents from Kennedy to Giancana and that it involved the "elimination" of Castro.

She also claimed that she had become pregnant by Kennedy, a pregnancy ended with an abortion arranged by Giancana.

In the mid-1970s, she married golf pro Daniel Exner. They separated in 1985. Survivors include a son; two brothers; and a sister.