Crillon joined the "Régiment du Roi" (King's Regiment) in 1734, aged 16, as a "lieutenant en second" and participated in France's Italian campaign during the War of the Polish Succession. He was soon promoted to "lieutenant en premier" and took part in a number of notable actions, including the Battle of San Pietro, under the command of the Marshal de Villars. He remained with the regiment until 1738 when he was promoted to the rank of colonel in the "Régiment de Bretagne" (Regiment of Brittany).