Born in Herzberg am Harz, Einem served in the Prussian army for much of his life when he was appointed Minister of War in 1903.

During his six years of service, Einem oversaw the reorganization of the German army building much of the military's heavy armament in preparation for modern warfare, specifically the introduction of the machine gun and modern heavy artillery.

Assigned to France, Einem succeeded Gen. Max von Hausen as commander of the Third Army in September 1914.

Successfully repulsing the French Champagne-Marne offensive from February–March and September–November 1915 respectively, Einem would take part in all three Battles of the Aisne and would hold Gen. Anthoine's 4th Army (under Gen. Philippe Petain's Center Army Group) during the Second Battle of the Aisne as part of the Nivelle Offensive from 16 April to 15 May 1917.

Einem's right wing units would also participate in Gen. Erich Ludendorff's Champagne-Marne offensive on 15–17 July 1918 supporting the east flank of the German 1st Army.