In 1940 Norway was attacked by Germany. During the subsequent fighting Hambro was a liaison officer for British forces in Western Norway, but later in the same year he fled via London to the United States. He was a guest scholar at the Northwestern University from 1941, and secretary-general in Norse Federation and editor of their magazine "Nordmanns-Forbundets Tidsskrift" from 1941 to 1943. He then returned to London to work in the Norwegian Ministry of Foreign Affairs-in-exile until the war's end. He was decorated with the Defence Medal 1940–1945.