[1] On 1 April 1914, Hartmann joined the Königlich Preußischen Kadettenkorps (Royal Prussian Cadet Corps) in Oranienstein near Diez.

He initially served in the Vorkorps (pre corps) before he transferred to the Hauptkadettenanstalt (Main Military Academy) in Berlin-Lichterfelde on 1 April 1917.

Following his graduation he began his naval career with the Reichsmarine of the Weimar Republic on 1 April 1921 as a member of "Crew 21" (the incoming class of 1921).

[2] It began with basic military training (1 April – 30 September 1921) in Stralsund at the Baltic Sea and weapons courses at the Naval Academy at Mürwik.

In 1944 Hartmann became Führer der Unterseeboote Mittelmeer ("Commander of U-boats in the Mediterranean") and in this post received the Knight's Cross with Oak Leaves.

[7] As commander of U-37 and U-198 Werner Hartmann is credited with the sinking of 26 ships for a total of 115,337 gross register tons (GRT).