During World War I, Duclos fought in the Battle of Verdun, where he was wounded. He was captured at Chemin des Dames, and remained a prisoner of war for the remainder of the war. In 1920, he joined the newly formed French Communist Party. He rose to the Central Committee in 1926, and defeated Léon Blum in the elections for deputy in the XXe arrondissement. He was named head of the propaganda section of the Party in 1936, and was elected to Vice-President of the French National Assembly.