After college, he took a job with the United States Department of War in Washington, D.C. as a civilian economic analyst and intelligence specialist.
After World War II, in 1945, De Palma joined the United States Foreign Service.
In 1966, President of the United States Lyndon B. Johnson nominated De Palma as Assistant Secretary of State for International Organization Affairs, and, after Senate Confirmation, De Palma held this office from February 11, 1969 until June 20, 1973.
De Palma left government service in 1973, becoming international analysis unit director of ITT Corporation.
In retirement, De Palma was president of the American and international committees of the Community of Democracies.