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. 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 not to put off attacking the Continental Army on the night of January 2, 1777. Now a Brigadier-General, Erskine accompanied British forces as second in command under General William Tryon with the assistance of General James Agnew on an inland raid against Patriot supply depots in Danbury, Connecticut. After successfully destroying Patriot supplies, the British forces engaged and defeated Continental Army Generals David Wooster, Benedict Arnold, and Gold S. Silliman and Patriot militiamen in the Battle of Ridgefield. Erskine served in the Philadelphia campaign and then, following Sir William Howe's resignation in 1778, continued as Quartermaster-General under Henry Clinton.

Erskine went on to lead troops at the Battle of Monmouth and after that campaign was given command of the eastern district of Long Island. Erskine's last active duty in North America was commanding five infantry battalions and a cavalry squadron in pursuit of the Continental Army which was thought to be moving to Virginia in November 1778. Promoted Major-General on 19 February 1779, Erskine sailed for London the same year. Erskine became colonel-in-chief of the 26th Foot (Cameronians) in 1782. On 28 September 1787 he was promoted lieutenant-general. He became a baronet in 1791. After the declaration of war with revolutionary France, he served on the staff of the Duke of York in the Flanders Campaign from late 1793. He again fought at Tournai soon after. Erskine commanded the 2nd (British) Line Corps in August. Erskine died on 19 March 1795, aged 67, and is buried at Torryburn, Fife.