De Moustier served in the French Army during the First World War and received the Croix de guerre 1914-1918 and the Légion d"Honneur. He was mayor of Cubry as his father had been before him and in 1928 again followed his father, this time to represent Doubs in the Chamber of Deputies, sitting with the Republican Federation. On the outbreak of the Second World War, he returned to service with the French Army, then aged 57.
After the signing of the armistice with Germany in June 1940, he was one of the 80 who voted against the grant of special powers to Philippe Pétain and the creation of the Vichy régime.
Moustier described the armistice as treason and Pétain as a traitor and was associated with the French Resistance. He was a district leader in the Organisation de résistance de l"armée, and provided assistance to the United Kingdom"s Secret Intelligence Service.
He was arrested by the Gestapo on 23 August 1943 and imprisoned first at Besançon and then at Compiègne, and was deported to Neuengamme concentration camp in the summer of 1944. He died there shortly after the camp was liberated.
He was posthumously made a member of the Ordre de la Libération.