 He was appointed Head of the Reconstruction Department and in 1943 was made a Counsellor.In this capacity he attended numerous international conferences, including those at Tehran, Yalta, Dumbarton Oaks, and Potsdam.After World War II, Jebb served as Executive Secretary of the Preparatory Commission of the United Nations in August 1945, being appointed Acting United Nations Secretary-General from October 1945 to February 1946, until the appointment of the first Secretary-General Trygve Lie.