He became active in the French Communist Party and in the trade union movement, rising to senior positions in both.

During World War I (1914–18), he was drafted into the navy in 1916, and assigned to the cruiser Guichen, which carried troops to the east.

He became a quartermaster and was one of the leaders of the mutiny aboard the Guichen, Jean Bart and France, on the Black Sea, on 26 June 1919.

He was arrested in Greece, tried by a military court in Brest and sentenced to five years of hard labor, part of which he served in the Dar Bel Hamri Penitentiary in Morocco.

Tillon returned to Rennes and worked as a fitter in different factories making agricultural machinery and chemical products.

He joined the French Communist Party, and was active in the Confédération générale du travail unitaire (CGTU) trade union movement.

After the armistice of 22 June 1940 Tillon took a different line from the other PCF leaders, who denounced the imperialist war, called for peace, and concentrated on opposition to the Vichy government.

Instead, in public statements in June and July 1940, Tillon spoke out for fighting for national liberation from the Germans.

Rather than limit armed action to Communists, it was decided to create a non-Communist organization, the Francs-Tireurs et Partisans Français (FTPF), under the Front national.

The National Military Committee of the FTP was set up in March–April 1942, headed by Tillon, who thus became the leader of the Communist resistance.

[3] Tillon remained a Communist leader after the Liberation of France and played an important role in politics as mayor, deputy and then minister.

Tillon remained Minister of Armaments under the governments of Félix Gouin and Georges Bidault from 23 January to 16 December 1946.

After the communist ministers were dismissed from Ramadier's government he and Laurent Casanova were made responsible for the PCF's military policy.

[3] In June 1970, Tillon coauthored a paper titled "It is no longer possible to remain silent" with Roger Garaudy, Maurice Kriegel-Valrimont and Jean Pronteau.