He was born in Trondhjem as the son of telegrapher Edward Diesen (1852–1909) and Johanne Christophine Stenersen (1855–1936).
He served in the neutral Royal Norwegian Navy during World War I and was promoted to Captain in 1917.
Here he edited the newspaper Hortens Avis from 1919 to 1920 and was a member of the executive committee of Borre municipal council from 1920 to 1923.
Two months later, Diesen embarked the ship HNoMS Fridtjof Nansen with which he fled the country.
He left his position as Commanding Admiral in August 1941, due to disagreements with the government, and served in the United States until 1944.