Thomsen served as Chargé d'Affaires at the Embassy of Germany in Washington, representing the German government in the U.S. from November 1938 (after the recall of ambassador Hans-Heinrich Dieckhoff) to December 11, 1941 (termination of relations after declaration of war).

[7] Like Dieckhoff, Thomsen suffered no illusions about the U.S. administration's policy towards Nazi Germany, and he sent warnings to the German government advising them of President Roosevelt's hostility.

[10] Thomsen reported to the German foreign ministry on June 12, 1940 that a "well-known Republican congressman" had offered to take a group of fifty isolationists to the convention in exchange for $3,000 (equivalent to $65,000 in 2023).

[13] [14] Just before the Pearl Harbor attack, Thomsen was involved in a curious attempt by William Donovan, the United States Coordinator of Information, to recruit him entirely to the American side.

Thomsen had been supplying information on German military strength and movements to Malcolm Lovell, a real estate developer involved in Quaker anti-war efforts.