Jessup served as assistant secretary-general of the United Nations Relief and Rehabilitation Administration (UNRRA) conference in 1943 and the United Nations Monetary and Financial Conference (the "Bretton Woods" conference) in 1944.
He was a technical advisor to the American delegation to the San Francisco United Nations charter conference in 1945.
He served from 1961 until 1970 as U.S. candidate for the International Court of Justice.
Jessup had been chairman in the Institute of Pacific Relations (IPR) at a time that organization was reflecting the Communist Party line.