Mercy entered the Austrian army as a volunteer in 1682. He won his commission at the great Battle of Vienna in the following year; and during seven years of campaigning in Hungary rose to the rank of Rittmeister. A wound sustained at this time permanently injured his sight. For five years more, up to 1697, he was employed in the Italian campaigns, then he was called back to Hungary by Prince Eugene of Savoy and won on the field of the Battle of Zenta two grades of promotion.