Erskine was posted to America in 1776 as a Brigadier staff officer under Clinton, seeing action in the New York campaign and commanding the 7th Brigade at Long Island 27 August 1776. In 1777 he was Quartermaster-General to Lord Cornwallis. Erskine is credited with advising Cornwallis on the occasion of the Battle of the Assunpink Creek (also known as 'Five Mile Run') not to put off attacking the Continental Army on the night of 2 January 1777. The Continental forces moved away by night, fighting the Battle of Princeton on 3 January.