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. Later, he was made chief of staff of the 12th Army that fought in the Battle of France. On 1 January 1940, he was promoted to lieutenant general and eight months later to General der Kavallerie. On 15 January 1941, Mackensen was made commanding general of III Army Corps, part of the 1st Panzer Army in Army Group South. On 27 July 1941 he received the Knight's Cross of the Iron Cross. Mackensen's forces were the first to reach Kiev at the First Battle of Kiev during the German invasion of the Soviet Union. In November 1942, when General Paul Ewald von Kleist was given the command of Army Group A, Mackensen took up leadership of the 1st Panzer Army in the Third Battle of Kharkov in March 1943. For his achievements in the Second Battle of Kharkov, Mackensen was honored on 26 May 1942 with the oak leaves to his knight's cross, and was promoted to colonel general (Generaloberst) on 6 July 1943.
