Trelawny served with the Coldstreams during the Raid on St Malo and the Raid on Cherbourg in 1758. He became a captain on 14 March 1762. Trelawny again saw service during the American Revolutionary War, in which he led the first battalion of the regiment. He fought at the Battle of White Plains in 1776, and was promoted to a colonel in the army on 6 September 1777. He again led the first battalion at the Battle of Monmouth, where he was wounded. He was promoted to first major of the regiment on 5 May 1780, later to major-general in the army, and was promoted lieutenant-colonel of the Coldstreams on 23 November 1785. He became Governor of Landguard Fort in 1788 and retired from the Guards on 25 May 1789. On 12 October 1793, he was promoted to lieutenant-general.