Between 1963 and 1968 Prandler was the counselor to the Hungarian delegation to the United Nations in New York. From 1976 he was appointed a special ambassador. From 1995 to 2002 he was a leader of the Hungarian delegation. He took part in the in the International Criminal Court. He was elected to the UN General Assembly and was the chairman of the legal committee. In 2005 UN General Assembly established a committee to investigate war crimes committed in the former Yugoslavia. Prandler was a key part of the process.