After the Occupation of France, Dormoy as a representative refused to approve providing full powers to Marshal Philippe Petain and the Vichy government.

He was elected in 1931 as representative of the Section française de l'Internationale ouvrière (SFIO) to the French National Assembly from the Allier département.

A member of the Popular Front's government, cabinet secretary to Léon Blum, he played a part in negotiating the Matignon Accords.

The French Police opened fire on a crowd protesting against a Croix-de-Feu rally in Clichy, after the event had degenerated into disorder.

In November 1937, Dormoy ordered the arrest of 70 members of La Cagoule after the police infiltrated the far-right organization.

Two years later, after the Fall of France, he was one of the minority of parliamentarians who refused to grant full powers to Marshal Philippe Pétain.