In addition to Chile, the New York native served as ambassador to strife-torn Cyprus, negotiated limits on nuclear tests and carried out the treaty giving Panama control of the Panama Canal, The New York Times reports.
A graduate of Harvard University, Popper traveled extensively in Latin America for the non-profit Foreign Policy Association.
He served in the U.S. Army tracking enemy activity in Latin America from 1942 until 1945 when he joined the State Department.
Popper's first overseas assignment for the department was negotiating nuclear issues with the Soviet Union.
During the Kennedy administration, he worked at the United Nations with U.S. Ambassador Adlai E. Stevenson.
Popper served as ambassador to Cyprus from 1969 to 1973 during a period of rising tensions between Greeks and Turks.
He became ambassador to Chile four months after General Augusto Pinochet overthrew socialist President Salvador Allende.