Söderman was a pioneer of modern criminology in Scandinavia, and the first head of the National Swedish Criminal Police Registry and Forensic Laboratories between 1939 and 1953.

Among the notable investigations he was involved in during this period were the 1931 Ådalen shootings, the 1932 Lindbergh kidnapping,[4] the 1933 German Reichstag fire, and the 1939 murder of Gerd Johansson.

[3] During World War II, he conspired with his international contacts, including Head of German Criminal Police Arthur Nebe, to invade Germany and arrest Adolf Hitler.

The plan, which would have required some 10,000 British paratroopers, was presented to Winston Churchill who, although initially interested, found it too risky and turned Söderman down.

[5] During the last part of World War II Söderman was in charge of the education of Danish and Norwegian police troops in exile in Sweden.

[5] Söderman co-wrote Modern Criminal Investigation in 1935 with New York City Police Department Chief Inspector John J. O'Connell.

Criminologist Leif G. W. Persson, a self-professed admirer of Söderman, has described him as "a Swedish Indiana Jones", and argues that his "larger-than-life" persona might have been better suited for the United States than Sweden.