Popper worked in Army intelligence at a base in Miami where he traced Axis power movement and influence in Latin America.

He served as Director of the Office of Atlantic Political and Military Affairs from 1962 to 1965 alongside Adlai E. Stevenson, then ambassador to the United Nations.

His tenure began in February 1974, four months after General Augusto Pinochet overthrew the democratically elected president Salvador Allende in the 1973 Chilean coup d'état, with tacit support from the US government.

After spending three years as ambassador to Chile he finished his diplomatic career as special representative for Panama Canal Treaty Affairs.

In semi-retirement David Popper served as the ghost writer for former UN Secretary-General Kurt Waldheim's memoirs, entitled In the Eye of the Storm.