Born in 1893, Matthias Kleinheisterkamp enlisted in the Prussian Army in 1914 and served on both the Western and the Eastern Fronts of World War I. During his service in the war, he was awarded both classes of the Iron Cross and a silver grade Wound Badge. Following the war, Kleinheisterkamp joined the paramilitary group Freikorps and then served in the Reichswehr. He joined the Allgemeine-SS in November 1933, membership number 132,399.