From 1940 to 1945 Hjelmtveit was exiled in London together with the rest of Nygaardsvold's Cabinet, because of the occupation of Norway by Nazi Germany. Hjelmtveit later defended the cabinet's actions on 9 April 1940, when they fled from the invading Germans. The war memoirs "Vekstår og vargtid" came in 1969. While exiled he wrote the book "Education in Norway", published 1946. Between the war's end (8 May 1945) and Hjelmtveit's return to Norway, Alv Harald Helland and Terje Wold filled in as Minister of Education and Research. He stepped down in June 1945, when Gerhardsen's First Cabinet took over. On 1 December 1945 he became County Governor of Aust-Agder, a post he held until 1961. He headed the Norwegian delegation to found UNESCO in London in 1945, and also headed the delegation for UNESCO's 1st General Conference in Paris in 1946.