Juan Antonio de Courten joined the Spanish army at the age of 12 and served in the Walloon Guards Regiment. He became an ensign on 11 January 1744 while in garrison at Rimini. He participated in the War of the Austrian Succession, fighting at the Battle of Velletri on 12 August 1744 and the Battle of Piacenza on 16 June 1746. He was also present at the sieges of Alessandria, Tortona, and Valenza during this period. On 13 August 1746 he became a second lieutenant. Promotion to first lieutenant came on 22 September 1754 and the following year he was appointed adjutant major of an artillery detachment.