 He became an ensign of the 5th Company of the same regiment on 21 May, and was promoted to second lieutenant on 11 November.He was commissioned as first lieutenant of the 3rd Pennsylvania Regiment in April 1777, and subsequently joined the 1st Mounted Troop of Lee's Legion.During his service with Lee's Legion Armstrong was captured by British dragoons led by Major John Coffin, in Dorchester, South Carolina, on 30 December 1781, along with four privates under his command.