On 9 April 1940 Norway was attacked by Nazi Germany. Hindahl followed the cabinet as they fled the capital. Unlike the cabinet Nordahl did not flee the country in 1940. After protests against the new Nazi authorities Nordahl was arrested in November 1940, but later released. The Confederation of Trade Unions leadership in Norway was filled with other people, perceived as more cooperationist—Nazis were however not installed until November 1941. Nordahl fled to the United Kingdom in September 1941 and established a leadership-in-exile. He was also a board member of Norges Bank-in-exile from 1941 to 1945. He is regarded as the legal Confederation of Trade Unions leader even through the war years. From 1945 to 1965 he was the undisputed leader. He was also elected twice to the Parliament of Norway, in 1957 and 1961. For both terms he was a member of the Standing Committee on Foreign Affairs and Constitutional Affairs and the Enlarged Committee on Foreign Affairs and Constitutional Affairs.