Hamann was of Baltic German parentage. Trained as a chemist, he had difficulties finding a job due to the Great Depression. He joined the SA in August 1931, the Nazi Party in December 1932, and the SS in July 1938. He served in the Luftwaffe during the invasion of Poland and Battle of France as a paratrooper ("Fallschirmjäger"). He returned to Berlin where he joined the SS and completed training courses. In March 1941, he was promoted to SS-"Obersturmführer" (first lieutenant). After the Nazi invasion of the Soviet Union, Hamann organized and commanded "Rollkommando Hamann" which killed at least 39,000 Jews in various locations across Lithuania and 9,102 people, almost all of whom were Jews, from the Daugavpils Ghetto. Hamann's superior, Karl Jäger, documented these killings in the Jäger Report. Nevertheless, Martin C. Dean estimates the death toll of "Rollkommando" Hamann as an estimated 60,000 Jews in Lithuania alone.