Born in 1898, Mikosch volunteered for the army of Imperial Germany in 1914. After two years service in World War I, he received a commission in an engineer battalion. After the war, he joined the police force where he remained for several years until he rejoined the German Army in 1935 as an engineer. With the rank of "Oberstleutnant" (lieutenant colonel), he commanded the 51st Engineer Battalion which fought in the Battle of Poland in 1939 and assisted in the capture of Fort Eben-Emael in Belgium on 11 May 1940. Shortly afterwards, he was awarded the Knight's Cross of the Iron Cross.