[4] Her mother was the elder sister of the composer Francis Poulenc and was herself a pianist and voice student of Jeanne Raunay and Claire Croiza.
In 1933 she met fellow student Jacqueline Bernard, who became a lifelong friend and later married Poulenc's then lover, Raymond Latarjet.
Poulenc wrote to Long that Manceaux was aiding him in the composition of his Concerto for Two Pianos,[8] explaining that she assisted him in rehearsing "certain passages a thousand times.
[20] Manceaux's assistance proved especially invaluable while Poulenc composed and attended to the rehearsals for Dialogues des Carmélites in 1957.
[23] Manceaux was described by her friend Jacqueline Latarjet as a fine musician, a charming woman, but a victim of an excess of modesty who lacked confidence in herself despite her qualities.
[23] Poulenc died on 30 January 1963 in his Paris apartment following weeks of illness;[24] he had suffered a mild heart attack the previous August.
The following March, Manceaux travelled to Milan to supervise La Scala's new production of Les Mamelles de Tirésias conducted by Nino Sanzogno.
"[26] The next month, she travelled with Pierre Bernac and Suzanne Peignot to New York City to attend the premieres of the Clarinet Sonata and Sept répons des ténèbres on 10 and 11 April respectively.
According to Marcel Schneider, who was by Manceaux's side at her death, her last words were: "My only consolation is knowing that I am going to meet Francis again, but at the same time it is hard to die when one is still young!