Born in Lyon, Aubert excelled in his studies, but he left school to complete an apprenticeship as a dyer, his father's trade.
This led him to join the French Communist Party (PCF), and he was soon appointed as the secretary of the CGTU's textile union.
He led the joint negotiations on wages in the textile industries, and once the CGTU merged into the General Confederation of Labour (CGT), he was a leading figure on the left of the union.
He was called up for service during World War II, in a tank regiment, was captured by the German Army in 1940, but managed to escape the following year.
This enabled him to become a director of a company making belts, and provided cover for him printing materials for a Lyon-based resistance group.