Mackensen wsa born in Haus Leipnitz in the Kingdom of Saxony, part of the empire of Prussia. He was a veteran of the Franco-Prussian War and was taught by Alfred von Schlieffen in the arts of strategy, and was already 65 by the time that World War I began. He fought in the Battle of Gumbinnen when the Russians invaded East Prussia in 1914 and later at the Battle of Tannenberg, and fought in East Prussia and Poland until 1915.
That year, he was transferred south to assist the German, Austro-Hungarian, and Bulgarian forces fighting the Serbians. He defeated French, Russian, and Serbian forces and followed up this success in 1916 with the Romanian Campaign, but failed to take the whole country. He failed to destroy the Romanian Army after the Kerensky Offensive's failure in 1917, missing a perfect opportunity to win the Eastern Front war, and was captured in Hungary.
After the war, he remained a committed monarchist and opposed Adolf Hitler's atrocities in the Night of the Long Knives in 1934 and attended Kaiser Wilhelm II of Germany's funeral in 1941. His son Eberhard von Mackensen, however, would serve as a Wehrmacht general during World War II.