In 1934, she married Paul Vaillant-Couturier, founder of the Republican Association of Ex-servicemen, a communist and chief editor of L'Humanité, who mysteriously died in 1937.

Attached to the Vu team as a photographer but also as a German speaker, she took part in an investigation in Germany into the rise of Nazism and travelled there in 1933, two months after Adolf Hitler came to power.

Vaillant-Couturier participated in the Resistance and helped produce clandestine publications, including leaflets such as: l'Université Libre (first issued in November 1940); Georges Politzer's pamphlet Sang et Or (Blood and Gold) which presented the theses of the Nazi theorist Alfred Rosenberg (November 1941); and a clandestine edition of L'Humanité with Pierre Villon (her second husband, whom she married in 1949).

Vaillant-Couturier was in Auschwitz for 18 months, where she witnessed the genocide of the Jews and the Gypsies and took part in the international clandestine resistance committee of the camp.

According to a June 16, 1945 article in Le Monde, “Each day, this magnificent Frenchwoman makes the rounds, uplifting courage, giving hope where it is often but illusion.

The newspaper, at the time closely related to the PCF, had attacked Rousset after he compared the Soviet Gulags to Nazi concentration camps.

A witness at the Nuremberg Trials,[4] she said later that “by telling of the sufferings of those who could not speak any more, I had the feeling that, through my voice, those who they had tortured and exterminated accused their torturers.” However, she returned from the trials “shocked, worried,” “exasperated by the procedure,” and particularly denouncing the absence from the dock of the leaders of businesses like Krupp, Siemens, and IG Farben, which had greatly profited from the economic exploitation of deportees.

[citation needed] In 1964, Paul Rassinier, one of the first Holocaust deniers, and a critic of the verdict of the Nuremberg trials, accused her of having survived the camps by stealing from other prisoners.

Geneviève de Gaulle-Anthonioz declared to the bar of witnesses “We entered the infirmary buildings not to hide, but because we needed courageous German speaking comrades.