After the war, he graduated and in 1920 he entered politics.
During the World War I, he served in Alpine regiment as an artillery officer After the war he re-entered politics, and became a deputy prime minister and in 1945, a minister of Defense. In January 1947, Brosio became the Italian ambassador to Soviet Union and got involved with the peace treaty negotiations between the countries.
In 1952 he became ambassador to the United Kingdom, to the United States of America in 1955 and from 1961–1964 to France.
On May 12, 1964 the North Atlantic Treaty Organization council chose Brosio to succeed Dirk Stikker as a secretary general. He resigned September 3, 1971.
Brosio died in Torino. He was the uncle of singer and television presenter Vanna Brosio.
Later his political activity was barred because of his opposition of Fascism.
Member Liberal Party, central secretary Liberal Revolution movement, 1922-1925, secretary-general Party, 1944-1945. Member underground National Liberation Committee, 1943-1944. Member of national council, national directorate Liberal Party.
Served with Alpini Corps, Italian Army, 1915-1918.
Married Clotilde Brosio, October 1, 1936.