"I will love you until the end of me," he writes to Anne Pingeot in one of more than 1,200 letters to be published next Thursday. "Feelings that I never knew are aroused with you," another reads.
The affair was hidden in plain sight to reporters who covered Mitterrand's 1981-95 presidency, bound by an unwritten but strictly observed code of respect for the private lives of public figures.
As a result, the public at large was oblivious to the Socialist leader's double life as head of state and a man crazy in love with a woman nearly 30 years his junior.
"Lettres a Anne" (Letters to Anne) will see the light of day more than 20 years after Mitterrand's death and five years after that of his wife Danielle, the mother of their two sons, Jean-Christophe and Gilbert.
Transcribed for publication by Pingeot herself, the correspondence startles not just with its eroticism but with its literary quality.
"O the desire for your arms, your being, of the fire and the swell, the shout that leaves us on the edge of another world," Mitterrand wrote.
The letters chronicle an affair that starts out with timid formality, using the "vous" form for "you".
The first one dated October 19, 1962, accompanied a tome of Socrates and was addressed to "Mademoiselle Anne Pingeot", then aged 19 and legally still a minor.
Penned when Mitterrand was a 46-year-old senator, he wrote: "This little book will be the messenger to tell you of the faithful memory I keep of a few hours in a lovely summer."
The pair had met the previous summer at Hossegor, a southwestern seaside resort.
By then Mitterrand had been married nearly 20 years to Danielle.
It is not until the pair travel to Amsterdam together in May 1964 that the intimate "tu" form emerges, along with vignettes from their trysts.
"I love my hands that have caressed your body, my lips that have drunk of you," Mitterrand wrote in July 1964.
Six years later the writing becomes downright breathless.
"I love your body, the joy that flows in me when I hold your mouth, the possession that burns with all the fires of the world, the gushing of my blood in your depths, your pleasure that erupts from the volcano of our bodies, flames in space, burning."
It was not until the year before Mitterrand left office in 1995 that the existence of Mazarine, now 41, was revealed with an expose in Paris Match magazine.