He then moved to the security police, where he stayed until 1935 when he re-entered the army.
After the war, Mülverstedt joined the Sicherheitspolizei on 15 September 1919 (later redesignated the Schutzpolizei) as a Hauptmann, and was assigned to posts in Berlin, Eiche, and Erfurt.
Beginning in 1939, after the German occupation of Poland, it became a re-education camp of the German security police (Arbeitserziehungslager der Sicherheitspolizei Warschau).
The goal of the operation was to "liquidate" Franz Bürkl, a notorious Sicherheitspolizei officer who had been sentenced to death by the Polish Special Courts for the murder of at least several dozen people.
In November 1940 he succeeded Walter Stahlecker in the dual command of the SD and Sicherheitspolizei for Norway and Oslo, reporting to Reinhard Heydrich and Ernst Kaltenbrunner in Berlin and Josef Terboven in Norway.
The Estonian International Commission for the Investigation of Crimes Against Humanity rests the responsibility for such crimes mainly on 2.5–4 % of the Estonian Omakaitse civil defence units and the Estonian Security Police.