De Lancey commanded the advance of the right column of the British Army under Sir Henry Clinton and Sir William Erskine at the Battle of Brooklyn, served at the capture of New York and the Battle of White Plains.
De Lancey then went upon the staff as deputy quartermaster-general to the force sent to South Carolina, and after serving at the capture of Charleston he became aide-de-camp to Lord Cornwallis, and eventually succeeded Major John André as adjutant-general to the army at New York.