Davis began his Foreign Service career with an assignment in Prague in 1947, followed by postings in Florence, Rome and Moscow, before returning to the U.S. in 1956 to work at the Soviet Desk at the State Department in Washington, D.C. His next foreign assignment was in Caracas, Venezuela, from 1960 to 1962. From 1962 to 1965, he served in the Peace Corps, first as Special Assistant to the Director, R. Sargent Shriver, and later Deputy Director for Program Development and Operations. He left the Peace Corps in 1965 to serve as the United States Envoy to Bulgaria (1965–1966). After his ambassadorship in Bulgaria, he served on the staff of the National Security Council in the White House, as President Lyndon B. Johnson's senior advisor on Soviet and Eastern European affairs, as well as the United Nations. In 1968, he went to Guatemala to serve as Ambassador to Guatemala (1968–1971), followed by service as Ambassador to Chile (1971–1973). He was ambassador in Chile during the presidency of Salvador Allende and through the coup that deposed him. Davis wrote a history of that period called "The Last Two Years of Salvador Allende". Upon his return from Chile, he held two positions at the assistant secretary level: as Director General of the Foreign Service (1973–1975) and as the Assistant Secretary of State for African Affairs in the Ford administration from 1975 to 1976. Davis resigned from the latter post over a policy difference with then-Secretary of State, Henry Kissinger, regarding covert action in Angola.