In the fall of 1941, Mary-Jo began studying medicine in Rennes and held an Ausweis (pass) that allowed her to circulate in closed areas of the coast where her mother had created the Bande a Sidonie network. Despite the danger, Marie-Jo began to carry messages that were slipped into her anatomical notebooks and transported to the Allies in England.
"Despite my age, I did not do it naively. The executions happened very quickly, the context was heavy, we measure risk."
Resistance activities on the coast were stopped in 1941, though Rennes network members continued to meet. Their new liaison "Georges", who was a double agent working for the Abwehr, renounced the group. Marie-Jo was arrested on 22 May 1942 and eventually transferred to Prison Health for questioning by the Gestapo. She was sentenced to death and deported to Ravensbruck on 26 July 1943 with her mother and 56 other French women. All 58 were placed in the 32 NN block (Nacht und Nebel).
On 2 March 1945 they were transferred to another NN at Mauthausen. They were released on April 21, and evacuated to Switzerland, by the International Red Cross as a result of negotiations between Himmler and Count Folke Bernadotte on behalf of the Swedish Red Cross and the "White Bus" operation
Mary-Jo returned to Brehat in May 1945 and resumed her medical studies. She married Paul-Henry Chombard de Lauwe with whom she had four children Marie, Noelle, Jean-Marie and Pascal.