Mackensen began his military service in 1869 as a volunteer with the Prussian 2nd Life Hussar Regiment (2. Leib-Husaren-Regiment Nr. 2). During the Franco-Prussian War he was promoted to second lieutenant and recommended for the Iron Cross, Second Class. After an interlude at Halle University, Mackensen formally entered the German Army in 1873, with his old regiment. In 1891 he joined the General Staff in Berlin, where he was heavily influenced by the new chief, Alfred von Schlieffen. From June 17, 1893 to January 27, 1898, Mackensen commanded the 1st Life Hussar Regiment (1. Leib-Husaren-Regiment Nr. 1), to which he became à la suite when he left command and whose uniform he often wore as a general. He was ennobled on January 27, 1899, becoming August von Mackensen. From 1901 to 1903, he commanded the Life Hussar Brigade (Leib-Husaren-Brigade), and from 1903 to 1908 he commanded the 36th Division in Danzig. When Schlieffen retired in 1906 Mackensen was regarded by some as a possible successor, but the job went to Helmuth von Moltke the Younger. In 1908, Mackensen took command of the XVII Army Corps, and commanded this corps until shortly after the beginning of World War I.
At the beginning of World War I Mackensen remained in command of XVII Army Corps as part of the Eighth Army under first General Maximilian von Prittwitz and then General Paul von Hindenburg and saw action in the battles of Gumbinnen and Tannenberg. On November 2, 1914 Mackensen took command of the Ninth Army from General von Hindenburg, who had been named Supreme Commander East (Oberbefehlshaber Ost). On November 27, 1914 Mackensen was awarded the Pour le Mérite, Prussia's highest military order, for actions around Łódź and Warsaw. He commanded the Ninth Army until April 1915, when he took command of the Eleventh Army and Army Group Kiev (Heeresgruppe Kiew), seeing action in Galicia, and assisting in the capture of Przemyśl and Lemberg. He was awarded oak leaves to the Pour le Mérite on June 3, 1915 and promoted to field marshal on June 22. After this campaign, he was awarded the Order of the Black Eagle, Prussia's highest-ranking order of knighthood. During this period, he also received numerous honors from other German states and Germany's allies, including the Grand Cross of the Military Max Joseph Order, the highest military honor of the Kingdom of Bavaria, on June 4, 1915.
He followed this up in 1916 with a temporarily successful campaign against Romania (under the overall command of General Erich von Falkenhayn). He was in command of a multi-national army of Bulgarians, Ottoman Turks, and Germans. Despite this, his offensives were very successful, breaking every army that faced his own. On January 9, 1917, Mackensen was awarded the Grand Cross of the Iron Cross, becoming one of only five recipients of this honor in World War I.
From 1917 on, Mackensen was the military governor of Romania. His last campaign was an attempt to destroy the Romanian army, which had been reorganised after the Kerensky Offensive was thrown back. But the attempt failed at the (Battle of Mărăşeşti), with both sides taking heavy losses. At the end of the war, he was captured by General Louis Franchet d'Espèrey's Allied army in Hungary (namely by the Serbian units) and held as a military prisoner until November 1919.
During the German elections of 1932 Mackensen supported Hindenburg over Adolf Hitler, but following the latter's accession to power Mackensen became a visible, if only symbolic, supporter of the Nazi regime. Mackensen's high-profile public visibility in his distinctive black Life Hussars uniform was recognized by the Hausser-Elastolin company which produced a 7-cm figure for its line of Elastolin composition soldiers (Figure #651/1)[See: Hausser Elastolin Spielzeg 1939-40 (toy catalog)]
Although Mackensen appeared in his black uniform at some public events presented by the German government or the Nazi party, he objected to the killings of Generals Ferdinand von Bredow and Kurt von Schleicher during The Night of the Long Knives purge of July 1934, and to atrocities committed during the fighting in Poland in September 1939. By the early 1940s Hitler and Joseph Goebbels suspected Mackensen of disloyalty but could do nothing. Mackensen remained a committed monarchist (notably, he appeared in full uniform at Kaiser Wilhelm II's funeral in 1941).