"It was heinous," she says. "It changed my view about sex for a long time."
Simon, now 70, says she told her older sisters what was happening at the time, but they didn't believe her – until more than 60 years later when Simon showed them an advance copy of her new memoir, Boys in the Trees, which will be released next week.
"Neither of them were at all aware of what had happened to me," she says. "I told them the first year that it was happening, and they thought I was just trying to be one of the bigger girls."
Even so, Simon says the information must have trickled down to their mother, who banned the boy from their home one summer – much to Simon's dismay.
"I was devastated because I thought I was in a romance. Which I think happens to a lot of girls," she says. "Your libido overpowers everything! You’re so libidinous even at the age of nine and ten. And sometimes there’s an outlet there. I bet in many more cases than we know about there is."
The singer says at the time she was focused on trying to make their secret time last – and didn't want to jeopardize it when she met with a child psychologist.
"I knew I had to keep it quiet. I wanted to keep it quiet because I wanted to keep it going," she says. "And even when Dr. Frunzhaufer thought that he could get to the bottom of it with 'Twinkle twinkle little star,' I didn’t want to tell him. I said, 'Yes he touched me on the chair. I was happy!' "
For more much on Carly Simon, pick up the new issue of PEOPLE, on newsstands Friday
As an adult, the singer went on to have dalliances with a number of famous men, including Warren Beatty – whom she has now revealed is the subject of the second verse of "You're So Vain" – Jack Nicholson, Mick Jagger, Cat Stevens and John Travolta.
She wed first husband James Taylor in 1973, and says that even though they haven't spoken in decades, she still "love[s] him so much." She is currently in a long-term relationship with Martha's Vineyard surgeon Richard Koehler, after ending her twenty-year second marriage to poet Jim Hart – whom she says came out as gay – on good terms in 2007.