Until 1940, Dr. Twigt held various posts in the private shipping business in Rotterdam, and from 1940 to 1949, he was an official in the Ministry of Economics of the Netherlands. He joined ICAO in 1949 and was Deputy Chief, Administration and Finance, when he left the Organization in 1956 to become Director of Administration of the UN Relief and Works Agency (UNRWA). Dr. Twigt was Chief Administrative Officer of the UN Congo Operation in Leopoldville from 1962-1963. Prior to that, he had been Chief Administration Officer of the UN Emergency Force in Gaza. He was Associate Director of Administration of the United Nations Technical Assistance Board until he assumed his post as Secretary General of ICAO on 1 August 1964. He served in that capacity for a period of six years. He died on 1 May 1993.