In 1972, Carly Simon Landed Her Signature No. 1 Hit—Thanks to Uncredited Vocals From Rock Royalty

Long before listeners began debating the inspiration behind Carly Simon’s signature hit, “You’re So Vain,” another mysterious guest star was standing right beside her in the studio.

Rolling Stones frontman Mick Jagger landed an uncredited backing performance that helped turn the 1972 song into a runaway smash. “You’re So Vain” made its Billboard Hot 100 chart debut in December 1972, and by January 1973, the song was No. 1, where it remained for three weeks.

Jagger was never actually supposed to sing on the track. That spot originally belonged to legendary singer-songwriter Harry Nilsson for the tune included on Simon’s No Secrets LP.

She told CBS This Morning that Jagger’s involvement on “You’re So Vain” was “kind of chance in a way.” Simon began, “I was in London, it was 1972, and he happened to call at the studio while I was doing the background vocals with Harry Nilsson.”

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“There was just a message that Mick was on the phone. I took the call, and he asked me, ‘whatcha doing?'” she continued. “I said, ‘We’re doing some backup vocals on a song of mine. Why don’t you come down and sing with us?'”

“Mick, Harry, and I stood around the mic singing, ‘You’re so vain, you probably think this song is about you,'” Simon recalled. “Harry was such a gentleman. He knew that the chemistry was between me and Mick in terms of the singing.”

She concluded, “And so he sort of bowed out, saying, ‘The two of you have a real blend. You should do it yourselves.’ And that’s how it happened.”

The performers did have chemistry that went beyond that recording session. Simon discussed their friendship in her 2015 memoir Boys in the Trees, and a passage discussing the Rolling Stones rocker was excerpted by PEOPLE Magazine.

“If Mick could have his way, it would be Romeo and Juliet tragic,” she wrote. “We couldn’t have each other.”

Simon explained that the night before her wedding to James Taylor, she received a call from Jagger’s wife Bianca.

“She told James that he shouldn’t marry me because her husband and I were having an affair. She muttered some things I couldn’t believe she was saying,” she wrote.

“Then James said, ‘I’m sure that’s not true. Carly has told me about it and it’s not what you think. I trust my wife-to-be. I trust Carly,'” she concluded.

Beyond its secret backing vocalist, the single sparked endless speculation over its subject, with fans theorizing it was written about everyone from James Taylor and Warren Beatty to David Bowie and Kris Kristofferson.

Simon admitted to People Magazine in 2015 that “the second verse is Warren.” The singer and songwriter also told the BBC, “Now, that doesn’t mean that the other two verses aren’t also about Warren. It just means that the second one is.”

She dropped a clue about whether the song referenced ex-husband Taylor in a 1973 interview with Rolling Stone. “It’s definitely not about James, although James suspected that it might be about him because he’s very vain.”

While the debate over who inspired “You’re So Vain” remains part of classic rock lore, the track’s true magic lies in that spontaneous London session. Over 50 years later, the blend of Carly Simon and Mick Jagger’s voices remains an unbeatable piece of 1970s music history.

Related: 1977 Sultry Carly Simon Hit Was Blocked From Reaching No. 1 by a Song People Loved to Hate