Following his compulsory military service, he studied history, German and sport at the universities of Tübingen, Cologne and Freiburg.

From 1990 to 1991, Hartmann worked as a consultant at the Political Archives of the German Foreign Office in Bonn, where he was a member of the international historical commission on the Akten zur deutschen auswärtigen Politik 1918–1945.

Since 1993, Hartmann has been research fellow at the Institute of Contemporary History in Munich/Berlin, where he was, among other functions, deputy editor-in-chief of the journal Vierteljahrshefte für Zeitgeschichte from 1998 to 2012 and director of the research project 'Wehrmacht in der nationalsozialistischen Diktatur 1933–1945’ (The Wehrmacht in the National Socialist dictatorship, 1933–1945) from 1999 to 2009, from which his study Wehrmacht im Ostkrieg also emerged and which was concluded with the edited collection Der deutsche Krieg im Osten 1941–1944.

From March 2012 to May 2015, Hartmann was project supervisor of the critical edition of Adolf Hitler’s work Mein Kampf.

As a lieutenant colonel of the reserve, he is currently lecturer at the Military Academy of the Bundeswehr (General Staff College) in Hamburg.