Mackensen joined the Imperial German Army in 1906, where he became a Fahnenjunker (officer candidate) in the XVII Corps stationed in Danzig, and was promoted to lieutenant on 22 March 1910.

Following the armistice in 1918 ending the war, Mackensen remained in the army (now the Reichswehr of the Weimar Republic) where he served as chief of the 1st squadron of the 5th (Prussian) Rider Regiment in Belgard, but in 1919 also joined a right-wing paramilitary Freikorps and fought in the Baltic states.

At the beginning of World War II in September 1939, Mackensen served as the chief of staff of the German 14th Army in the invasion of Poland.

For his achievements in the Second Battle of Kharkov, Mackensen was honoured on 26 May 1942 with the oak leaves to his knight's cross, and was promoted to colonel general (Generaloberst) on 6 July 1943.

After the Allies broke through the Winter Line during Operation Diadem, Kesselring fired Mackensen for disobeying orders and the latter retired from active service in the army in the summer of 1944.

[4] On 24 March 1944, SS members shot and killed 335 Italian civilians in the Ardeatine Caves massacre in retaliation for the deaths of 32 German police troops in a bomb attack.

After his release, Mackensen lived a secluded life in Alt Mühlendorf (now in Warder in Rendsburg-Eckernförde district) near Nortorf in Schleswig-Holstein, West Germany.