Tillon joined the Communist Party and became a trade union activist in the chemical industry. In 1936 he was elected to parliament as a representative of the working-class area of Aubervilliers.
In the summer of 1940 Tillon joined the French Resistance and the following year became one of the leaders of the Frances-Tireurs Partisans (FTP). Tillon organized the FTP along military lines with local, regional and national command structures.
After the D-day landings took place Tillon attempted to organize a communist revolution. However, under instructions from Joseph Stalin in the Soviet Union, Maurice Thorez and other leaders refused to cooperate.