Practiced as architect, 1913-1917. Assistant naval attaché Havana, 1917-1919. 3d secretary embassy Madrid, Spain, 1920-1922.
Charge d'affaires Port au Prince, Haiti, 1922-1924. 1st secretary American embassy, Brussels, Belgium, 1924-1927. Director ceremonies at White House, Washington, 1924-1930, chief Division of International Conference and Protocol, 1928-1930, 31-35.
Counselor to Commission for Study of Haiti, 1930. 1st secretary American embassy, London, 1930. Secretary delegate 4th Pan-American Commercial Conference, Washington, 1931.
Secretary general American delegate 1st stage General Disarmament Conference, Geneva, 1932. Also assistant 1st meeting expert's preparatory committee International Monetary and Economic Conference, Geneva. Secretary general American delegate International Monetary and Economic Conference, London, 1933.
Secretary general American delegate 7th International Conference American States, Montevideo, 1933. Special assistant to secretary of State, 1934, chief Division Western European Affairs, 1935-1937, advisor on political relations, 1937-1944, director Office of European Affairs, 1944, appointed assistant secretary of State, 1944. Member United States group Dumbarton Oaks Conversations on International Organization, Washington, 1944.
Advisor United States delegate United Nations. Conference, San Francisco, 1945. Chief political advisor Berlin Conference, 1945.
Deputy to the United States member meetings of Council of Foreign Ministers, London, Paris, New York City, 1945-1946, Paris Peace Conference, 1946. Ambassador to Italy, 1946-1952, France, 1952-1953, Spain, 1953-1955, Brazil, 1955-1956. With United States Naval Reserve, 1917-1919.
Dunn was chief political adviser to the Berlin Conference in 1945. Deputy at the American meetings of the Council of Foreign Ministers in London, Paris and New York in 1945-1946. He was once called a "fascist" by Eleanor Roosevelt for his views on colonial matters.
In 1946 he was a member of the delegation at the Paris Peace Conference. He was a governor of the Metropolitan Club and a member of the Knickerbocker Club, the River Club, the Regency Club and the Whist Club in New York, and of the Alibi Club in Chevy Chase, Maryland (U.S.)
Son of John Henry and Mary Emma (Delaney) D. M. Mary Augusta Armour, December 8, 1914. Children: Marianna Armour Dunn, Cynthia Louise Dunn Estherichner.