Following 9 April 1940, when Norway was invaded and subsequently occupied by Nazi Germany, Bull took part in various negotiations, being dispatched to non-occupied Stockholm in Sweden. He was ambassador in Stockholm from 1940 to 1945, taking over as chargé d'affaires after the death of Johan Wollebæk in October 1940. He was recognized as minister by the Swedish authorities from 1942. He played an important role at the Norwegian legation in Stockholm during World War II, when there was a colony of more than 50,000 Norwegian refugees in Sweden.