She went to work for a publishing company in Paris.
When Henri-Philippe Petain signed the armistice with Germany in June, 1940, she joined the French Resistance and worked under Georges Loustaunau-Lacau. After Loustaunau-Lacau was arrested in May 1941, Marie-Madeleine took over the running of the unit.
The group concentrated on obtaining intelligence information about the German armed forces and sending it to Britain. The British military authorities were so impressed with the quality of this information they sent her a wireless operator in August, 1941. It turned out this wireless operator was a double agent and Marie-Madeleine and several of her unit were arrested by the Gestapo. Unlike most of her colleagues Marie-Madeleine managed to escape and was forced into hiding.
After sending her children to live in Switzerland she now concentrated on helping to develop a network for returning shot down airman to Britain.
In July 1943, MI6 decided it was too dangerous for Marie-Madeleine to remain in France and brought her to England with her latest batch of airmen. She then ran her network from a house in Chelsea. Soon after the D-day landings Marie-Madeleine returned to France where she was soon captured by the Gestapo. But once again she managed to escape and get back to Allied lines.
During WWII 438 members of Marie-Madeleine's network were executed. In 1973 Marie-Madeleine wrote about her wartime experiences in Noah's Ark (thus called by the Germans because agents took the names of animals and birds). She died on 20 July 1989.