Marsha Hunt, the young Rolling Stone's apparent muse, is now 66 and living in France, broke and unable to pay her bills.
The letters, which were penned during the summer of 1969, are said to show a sensitive side of Jagger, who was then in his mid-20s.
"The letters speak for Mick at an incredible juncture in our lives," Hunt said. "The summer of '69 was the end of a whole era of revolutionary spirit. We didn't know it was about to die."
The couple's relationship, which produced a daughter, Karis, was kept under wraps until 1972.
Hunt, who is African-American, was a fixture on the 1960s social scene in London, where she was one of the few prominent black women.
The letters are expected to fetch up to $160,000. For Hunt it's not only about the money, but also preserving rock and roll history.
"Someone, I hope, will buy those letters, as our generation is dying," she said. "And with us will go the reality of who we were and what life was."