Carly Simon admitted it took years for her to finally speak out about her friendship with Jackie Kennedy Onassis during the last decade of Onassis' life.

The celebrated singer is releasing a new memoir on Oct. 22 titled, “Touched by the Sun: My Friendship With Jackie,” which details the former first lady’s thoughts about both of her husbands -- President John F. Kennedy and Aristotle Onassis -- as well as her two children.

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“Why am I writing about someone so private who disliked attention?” the 74-year-old told People magazine on Wednesday. “If someone affects your life as Jackie did mine, you can’t not write about her. [But] I withheld a great deal.”

Good friends Carly Simon and Jackie Onassis pose for a picture at Bunch of Grapes Bookstore on Martha's Vineyard, Massachusetts, 9/2/89.

Good friends Carly Simon and Jackie Onassis pose for a picture at Bunch of Grapes Bookstore on Martha's Vineyard, Massachusetts, 9/2/89. (Photo by Stephen Rose/Getty Images)

According to Simon, it was Onassis’ son John F. Kennedy Jr. who introduced his mother to her on Martha’s Vineyard back in 1983. The two remained close until Onassis’ death in 1994 at age 64.

“I could be neurotic, bohemian and all over the place; she always had to be so correct,” revealed the “You’re So Vain” singer. “I was who she wasn’t. I think she got a big kick out of that.”

While Onassis was 16 years older than Simon and notoriously private, it didn’t take long for her to open up.

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Former United States President John F. Kennedy and first lady Jackie Kennedy attend a dinner in honor of Andre Malraux, minister of state for cultural affairs of France, in Washington, in this handout image taken on May 11, 1962. November 22, 2013 will mark the 50th anniversary of the assassination of President Kennedy. REUTERS/Robert Knudsen/The White House/John F. Kennedy Presidential Library  (UNITED STATES - Tags: POLITICS ANNIVERSARY)    ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. FOR EDITORIAL USE ONLY. NOT FOR SALE FOR MARKETING OR ADVERTISING CAMPAIGNS. THIS PICTURE IS DISTRIBUTED EXACTLY AS RECEIVED BY REUTERS, AS A SERVICE TO CLIENTS - RTX15FKN

Former United States President John F. Kennedy and first lady Jackie Kennedy attend a dinner in honor of Andre Malraux, minister of state for cultural affairs of France, in Washington, in this handout image taken on May 11, 1962. (REUTERS/Robert Knudsen/The White House/John F. Kennedy Presidential Library)

Simon said Onassis even talked about President Kennedy’s adultery.

“[She] understood that he made some stupid mistakes,” said Simon. However, she told the outlet that Onassis brushed off the affairs because “she knew he loved her more, much more.”

Onassis also got candid about her second husband, Onassis, whom she married in 1968 — five years after Kennedy’s 1963 assassination.

“She said he was very sexy — he would sing to her on his yacht and take her great places to make love,” Simon recalled. “She knew marrying him was the bad-girl move. In a way, I never knew whether she was sticking up for her own decision or whether she really loved him.”

Millionaire shipping magnate Aristotle Onassis with his wife Jackie.

Millionaire shipping magnate Aristotle Onassis with his wife Jackie. (Getty)

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No matter what was occurring in Onassis’ love life, she remained fiercely protective of her son John Jr. and daughter, Caroline Kennedy. Simon said Onassis was particularly worried about her son.

“Jackie was very consumed with getting him on the right path,” Simon explained. “She didn’t approve of some of [the women] — she was horrified by Madonna.”

But Simon also stressed that Onassis was “incredibly funny.” She described how the two women attended a recording session with opera singer Placido Domingo in 1990. The next day, Simon received a gushing letter and photograph from him. But when Simon called Onassis to share the exciting news, she got an unexpected revelation.

(Original Caption) Mrs. Jacqueline Onassis was one of a group of prominent New Yorkers who joined the fight to save Grand Central Station during a meeting to fight construction of an office building atop the New York City landmark.

Jackie Kennedy Onassis in 1975. (Getty)

“She paused for a long time, and then she said, ‘Carly, did you really think it was from Placido?’” said Simon. “She’d written it up herself and disguised her handwriting! The practical joker in her was nonstop.”