Kempf joined the Imperial German Army on in 1905; following World War I, he served in the "Reichswehr" and later the "Wehrmacht". In October 1937 Kempf took command of the newly formed 4th Panzer Brigade; in January 1939 he was promoted to "Generalmajor". At the beginning of World War II in Europe, he took part in the invasion of Poland as commander of Panzer Division Kempf, which was also known as the "Panzerverband Ostpreußen" (Panzer Group East Prussia) of the 3rd Army under Georg von Küchler. As divisional commander, he received the capitulation of Fort Zakroczym, which was followed by Massacre in Zakroczym, at the conclusion of the Battle of Modlin. The division returned to East Prussia at the end of the Poland campaign, and Kempf was named commander of the 1st Light Division, renamed 6th Panzer Division, on 18 October 1939.