Thomas was promoted to cabinet in May 1915 when his position was upgraded to Under-Secretary of State for Armaments within the War Ministry. The vital importance of munitions production was recognised by the further promotion to Minister for Munitions at the end of 1916. Thomas retained his post in the Ribot government of 1917, but was replaced by his deputy, Louis Loucheur, in Painlevé's government which took office in September of that year. The socialists were unwilling to have Thomas continue in government. Earlier that year he had been sent as special ambassador to Russia and was instrumental in persuading Kerensky to undertake his offensive, the failure of which hastened the Bolshevik Revolution.
The role of Albert Thomas in the industrial mobilisation of France was highly important. He increased the labour force by giving employment to women and to foreign and colonial workers. He protected their rates of pay and tried to prevent the loss of skilled workers to the armies. He welcomed greater state intervention in industrial concerns, in order to increase output and keep prices within acceptable limits. Nonetheless, his acceptance of the overriding importance of maintaining supplies of good quality munitions and artillery pieces meant that he was in effect working as a capitalist within the industrial-military complex, despite his socialist affiliations. He achieved a great success in bettering the French output of munitions, despite the loss of France's industrial capacity in the occupied departements, but at the cost of tolerating the erosion of social legislation (for example, he removed limitations on the hours workers could work) in the interests of greater productivity. This was a contradiction which cost him the support of his party and, ultimately, his cabinet position.
Postwar, he returned to parliament as deputy for Jaurès' old seat and was elected Director of the International Labour Bureau. Pressure of work forced his resignation as deputy, but he remained a member of the French Socialist Party.