Mitrofanov was sent to study at the Military Academy for Mechanization and Motorization in February 1933 and upon graduation in June 1937 became chief of the 2nd staff department of the 5th Mechanized Corps of the Belorussian Special Military District. From October 1938, he served as chief of the 1st staff department of the 15th Tank Corps, formed from the 5th Mechanized Corps. In this capacity, Mitrofanov participated in the Soviet invasion of Poland before entering the Military Academy of the General Staff in December 1939, from which he graduated in 1941.