During the Italian Resistance period after the armistice with the Allies in September 1943, Brosio was the secretary of the Liberal Party and in 1944 was nominated minister without portfolio in the Bonomi government of 1944. Following World War II, Brosio became deputy premier in the first Ferruccio Parri government. In December 1945, Brosio was appointed defense minister in the government of Alcide De Gasperi, holding the post until July 1946.
Brosio began his diplomatic career in 1947 when he became ambassador to the Soviet Union. Fearing a Soviet reprisal, he was at first cautious about Italy's commitment to NATO, but he later embraced the idea of an Atlantic military defense structure. During 1952–1955 he served as ambassador to Britain, and from 1955 to 1960 he was Italy's ambassador to the United States. He then became ambassador to France, a post he held from 1960 to 1964. In March 1964 Brosio was appointed general secretary of NATO, a position he held until his resignation in 1971.
For his long and dedicated service to the Western alliance, Brosio received the Presidential Medal of Freedom from President Richard M. Nixon in 1971. Brosio died on 14 March 1980 in Turin. His diaries from his tenure in Moscow were published posthumously as Diari di Mosca (1947–1951).
Colombo, Emilio. "Manlio Brosio liberale atlantico." Nuovi studi politici 30(4) (2000): 83–86.; Schmidt, Gustave, ed. A History of NATO: The First Fifty Years. 3 vols. New York: Palgrave Macmillan, 2001.