Monckton commanded the 45th Foot from 25 July 1771 until 1772. He was wounded at the Battle of Long Island on 22 August 1776, while leading the 1st Grenadier Battalion in the army of Sir William Howe with the rank of lieutenant colonel. At time of the Battle of Trenton on 26 December 1776, he served as acting commander of Major General James Robertson's 1st British Brigade at New York City. After the Trenton disaster, he took command of an "ad hoc" brigade consisting of William Medows' 1st Grenadier Battalion, his own 2nd Grenadier Battalion, and James Ogilvie's 2nd Guards Battalion. Leading this force, he went to New Jersey with Lord Charles Cornwallis and fought at the Battle of the Assunpink Creek on 2 January 1777. The following day, George Washington outmaneuvered the British and defeated them at the Battle of Princeton. In the aftermath, Cornwallis made a rapid retreat to his base at New Brunswick, New Jersey.