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 the right-wing "Freikorps" paramilitary group and fought in the Baltic states. In 1925, Mackensen commanded the army transport department of the German General Staff of the Ministry of the Reichswehr in Berlin. After his appointment as a major on 1 February, 1928, he served from 1930 in the staff of the 1st Cavalry Division in Frankfurt (Oder), and while serving in this position was promoted on 1 October 1932, to lieutenant colonel