He was born in Kristiania as a son of Gerhard Olsen (1867–1949) and Emma Hansen (1872–1949).
When the occupation of Norway by Nazi Germany started, Gerhardsen fled to Sweden where he edited the publication Norges-Nytt at the Norwegian legation in Stockholm.
[1] He also chaired the Oslo branch of the Norwegian Labour Party and was known as an anti-Communist.
[1] In post-World War II Norway he built up a private intelligence service on behalf of the Labour Party, the so-called Alpha Network.
He released three books, including 1967's Einar Gerhardsen som en bror ser ham about his brother.