| Gunnar Jahn obtained the degree of Master of Law in 1907. After having worked as a solicitor in 1907-1908, he studied economics and statistics at universities in Heidelberg, Berlin and Paris. In 1910, he was employed at the Norwegian Central Bureau of Statistics, where he worked as secretary in 1911-1917. In 1920 he became Director of the Bureau, a position he held until 1945. From 1946 up to 1954 Jahn was Managing Director of the Norwegian Central Bank.
Gunnar Jahn belonged to the urban liberal faction of the Liberal Party (Venstre). He served as Finance Minister in a non-socialist (Venstre) government 1934-1935. During the German occupation of Norway 1940-1945, he was a member of the central leadership of the Resistance. He was imprisoned in 1944. After the German surrender in 1945, he became member of the Norwegian interim government. Jahn was a member of the Norwegian Nobel Committee in 1938-1966. He served as chairman of the committee from 1942 to 1966.