Malvy became a deputy in the Chamber of Deputies in 1906 as a Radical-Socialist, and served in several minor ministerial positions before the war. He became Minister of the Interior in Viviani's first ministry, retaining that post in Viviani's second ministry, both Briand's wartime ministries and the short-lived Ribot ministry; that is until the summer of 1917. He owed his continuance in government to his backing from the Radical-Socialists in the Chamber. If the union sacrée was to be maintained some ministerial posts had to be held by the left, especially in the absence of Joseph Caillaux whose political ally Malvy was.
At the outbreak of war Malvy resisted calls for the round-up of so-called subversives unionists, pacifists and other 'undesirables' whose names were listed in the Carnet B. He became involved in subsidising newspapers and came under intense criticism when it was learned that the Bonnet Rouge (one of the papers supported by Malvy) had been receiving German money to spread pacifist propaganda. When the paper's administrator, Duval, was arrested with a cheque on his person from a German banker, Clemenceau accused Malvy during a secret session of the Senate in July 1917 of 'betraying the interests of France'. When the paper's director was also arrested as a result of the subsequent investigation (which revealed the sale of the paper to a shadowy intermediary, Bolo Pacha), the paper was closed and its director died in prison.
Added to the civilian unrest of 1917 and the French Army mutinies, the discovery of French military documents in the Bonnet Rouge's offices meant the end for Malvy. As Minister of the Interior, he was blamed for not suppressing defeatist and pacifist agitators and publications with sufficient energy and he was forced to resign on 31 August 1917. In October he was charged with treason and tried by a special commission of the Senate the following year. Although acquitted of treason, he was found guilty of culpable negligence in the performance of his duties and banished for five years. His former directeur du cabinet was imprisoned, however, and Bolo and Duval received death sentences.
Despite this close association with men found guilty of serious crimes, he returned to theChamber of Deputies after the war and again became Minister of the Interior, briefly, in 1926.