Lord Charles Hay was born c. 1700, the third son of Charles Hay, 3rd Marquess of Tweeddale, and his wife Lady Susan Hamilton, the daughter of William Douglas, Duke of Hamilton. Lord Charles was the younger brother of John Hay, 4th Marquess of Tweeddale. Hay entered the army, being gazetted ensign in the 2nd Regiment of Foot Guards on 18 May 1722, and a captain in the 33rd Regiment of Foot on 14 May 1727. He was apparently present at the siege of Gibraltar in 1727, and in 1729 he was serving as a captain of the 9th Regiment of Dragoons. He took part as a volunteer in the armies of Prince Eugene of Savoy during the Rhine campaigns of 1734 in the War of the Polish Succession. He was elected as the member of parliament for Haddingtonshire in 1741 and on 7 April 1743 was given command of a company in the 3rd Foot Guards. He took part in the European campaigns of the War of the Austrian Succession, and was a virtual, if not actual, lieutenant-colonel of the 1st Regiment of Foot Guards at the Battles of Dettingen and Fontenoy.