He was assigned to duties in the border regions between Norway and the Soviet Union and was an intelligence officer in Finnmark during the Winter War, monitoring suspected Communist sympathizers.

He played an instrumental role in implementing the Holocaust in Norway, resulting in the murder of more than 700 Jews and the brutal mistreatment of many more; he was also known to take a relaxed view of legal process, and tolerated if not encouraged torture among his forces.

The car had just started to move when they opened fire with automatic weapons, instantly killing Marthinsen in the passenger seat and lightly wounding his driver.

Documents disclosed after the war indicate that the political leadership ordered the assassination to prevent Marthinsen from carrying through his plans to enlist Norwegian paramilitary forces to violently subvert the expected capitulation of Nazi Germany in Norway.

[5] The extent and severity of the reprisals shocked the Norwegian population and government-in-exile, resulting in a general moratorium against targeted killings of high-ranking Nazi officials.