The Truth Behind JFK's Mobbed Up Mistress — and What She Might Have Known About the Assassination

President John F. Kennedy‘s tendency toward extramarital romances has been well-documented in the decades since his assassination – but did one of his relationships play a direct role in his untimely death?

Judith Exner famously had a 2½-year affair with JFK, ending in 1962 – just one year before his assassination in Dallas.

Now – ahead of the 18th anniversary of her death at age 65 after a battle with breast cancer – PEOPLE is looking back on her stunning 1977 memoir, which was full of revelations about the president’s much-debated relationship with the mob.

Life Among the Stars

Exner, the daughter of an architect, grew up in relative style in Pacific Palisades, California, according to a 1988 PEOPLE profile. As a teenager, a short marriage to actor William Campbell introduced her to the Hollywood scene – where she eventually began living a glam singleton life.

It was during that era – in 1959 – that Exner reconnected with acquaintance Frank Sinatra in Hollywood, earning an invite to join the crooner in Hawaii for a November trip. A tumultuous affair began between the pair, and the duo remained friends even after it ended, with Ol’ Blue Eyes even inviting her to a Feb. 1960 concert at the Sands hotel in Las Vegas. It was there that Sinatra introduced Exner to Kennedy – then a Democratic candidate for president.

Exner’s infatuation with Kennedy was immediate, and for the next month the pair allegedly spoke on the phone almost daily. “He seemed very anxious to get together again. I was elated, almost giddy,” Exner told PEOPLE in 1988. “The world looked wonderful.”

They arranged to meet at The Plaza hotel in New York City on March 7, 1960, the night before the New Hampshire primary. It was then that their relationship was consummated.

Soon after, Exner was again spending time with Sinatra, attending his show in Miami Beach and mingling with the singer’s mobster friends, Joe Fischetti and Sam Giancana. Though she claimed to not know that Giancana was the “Chicago Godfather,” Exner told PEOPLE that his powerful pull over Sinatra was obvious.

Exner was soon positioned as a liaison: “I feel like I was set up to be the courier. I was a perfect choice because I could come and go without notice, and if noticed, no one would’ve believed it anyway.”

The Conduit

In April 1960, Exner was given her first assignment as a courier during a dinner at then-Sen. Kennedy’s Georgetown home while pregnant wife Jackie Kennedy was away on vacation. Kennedy and a lobbyist were allegedly concerned about securing votes in the West Virginia primary, and a decision was allegedly made to have Exner reach out to her new friend Giancana.

” ‘I think I may need his help in the campaign,’ Jack said,” per Exner. “He wanted the meeting as soon as possible and gave me a few dates that were good for him.”

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Exner eventually said she arranged around 10 meetings between Kennedy and Giancana, one of which she believed even took place inside the White House. Exner told PEOPLE that Giancana once bragged to her that Kennedy would never have been president “if it hadn’t been for his efforts on Kennedy’s behalf in Cook County, Illinois.” An overwhelming turnout for Kennedy in Cook County enabled him to carry Illinois by a slim 8,858 votes.

In addition, she transported secret envelopes between the two men, which she told PEOPLE she believed to be about the CIA’s collaboration with the Mafia to assassinate Cuban Premier Fidel Castro.

Dangerous Liaisons

As Kennedy’s focus on the presidency intensified, Exner’s relationship with him began to implode. Around the same time, she realized she was being followed by federal agents tasked with investigating Giancana. She said, “I called Jack immediately to tell him that the FBI had been to see me, asking all sorts of questions about Sam. I told him I had said I knew nothing about Sam’s business affairs. Jack reassured me. He said, ‘Don’t worry. They won’t do anything to you. And don’t worry about Sam. You know he works for us.’ ”

The affair ended later that year. Kennedy was killed the next.

Over a decade later, Exner was subpoenaed to testify in front of the Senate. She lied.

“If I’d told the truth, I’d have been killed. I kept my secret out of fear,” she told PEOPLE in 1988, adding, “Look what happened to Jack, and to Sam, who was murdered in his house while under police surveillance.”

Giancana was shot seven times in the head – just before he was to be called to testify. His killer never found. Another mafioso, John Roselli, did testify before the Senate about the CIA’s attempts to kill Castro. The next year, his body was found in an oil drum floating near Miami.

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“I became paralyzed with fear and started sleeping with a gun under my pillow,” admitted Exner.

So why speak out decades later?

“For the past 25 years I have been terrified to tell the truth about my relationship with Jack Kennedy. In fact, I’ve gone to great lengths to keep the truth from ever coming out, which is probably the only reason I’m alive today. With the exception of Sinatra, all the key figures involved in my story have been murdered,” she told PEOPLE almost 30 years ago.

But devastatingly ill with metastatic cancer, Exner – then 54 – said, “My doctor gives me about three years to live, and I want to put my life in order so that I can die peacefully. For that reason, I must now tell the truth.”

She would live much longer though, eventually losing her cancer battle in 1999.

Meanwhile, the mystery surrounding JFK’s shocking assassination — and whether the mob played a role in his death — endures.