Sønsteby was working as an accountant when occupation began and he came to join the Milorg Norwegian resistance network. Frustrated at their initial caution, he joined the pro-Nazi state police to gain intelligence and to travel.
Through this role he crossed into neutral Sweden, visited the British embassy in Stockholm and contacted the Special Operations Executive (SOE), a British organisation aiming to foster resistance abroad. Sønsteby would go on to be involved in numerous resistance efforts through the SOE group Norwegian Independent Company 1, and would coordinate many others in Norway.
His work included helping to destroy official records used by the occupiers to round up young Norwegians to send to the Russian front or into forced labour. Amongst his Company was Joachim Rønneberg, who had escaped by boat to Scotland in 1941, and who went on to lead the sabotage of the Vemork hydroelectric plant in Norway where the occupiers were producing heavy water. In so doing they managed to severely undermine the Nazis’ nuclear weapons production.
Sønsteby declined job offers from the British and Norwegian intelligence services following the war, stating that he “didn’t want any more war”. He moved to America to study at Harvard University before working in business for some years, later working at the Norwegian Home Front Museum.