Auguste Tilkens was born in 1869 into a large family in the Flemish coastal city of Ostend. He entered the Royal Military Academy in 1887 and became an artillery officer in the Belgian Army. Following the outbreak of World War I in August 1914, during German invasion of Belgium, Tilkens served in the Belgian forces and was decorated for valour at the Battle of the Yser. In 1916, with emergence of static trench warfare on the Western Front, he volunteered for service in Belgian Congo where fighting had broken out on the colony's border with German East Africa. He served under Charles Tombeur during the subsequent fighting in East Africa. In 1917, he returned to Europe where he became aide de camp to King Albert I. After the end of World War I in 1918, Tilkens remained in the army and rose through the ranks further, taking overall command of Belgium's artillery and, later, its Air Force. In 1927, he was promoted to the rank of lieutenant general.