In April 1940, after the German invation of Norway, Gudrun Martius made her way to Stockholm, where she worked a period for the Norwegian embassy. She eventually moved to London, where both her sister and her future husband Johan Georg Ræder were stationed at the embassy. Travelling by plane to Helsinki, and further with ship from Petsamo via Iceland to Scotland, she arrived in London on 7/8 August 1940. Following ongoing conflicts around Halvdan Koht, the Minister of Foreign Affairs in the Norwegian government-in-exile, Martius replaced Unni Diesen as secretary for Koht from end of August, until Koht's resignation in November 1940. Having married Ræder in 1940, she continued working for the Ministry, and from 1941 to 1943 she was personal secretary for the new Minister of Foreign Affairs, Trygve Lie.