Born in Phnom Penh, he had two famous elder brothers: Thiounn Thioeunn [ru], Minister of Health; and Thiounn Mumm, Ministry of Industry.
[1] In late December 1955, he returned to Cambodia, and was appointed deputy manager of "operations" of the royal railways.
[3][4] During the Khmer Rouge's regime, Thiounn Prasith worked as an interpreter in Ministry of Foreign Affairs.
[5] After the Pol Pot regime fell in the course of the Vietnamese invasion of 1979, he went to the United Nations in New York together with Sihanouk and Keat Chhon.
[2] After retirement, he was reported to be living in the New York suburb of Mount Vernon.