L'Enfant was wounded at the Siege of Savannah on October 9, 1779. He recovered and became a prisoner of war at the surrender of Charleston, South Carolina on May 12, 1780. He was exchanged in November 1780 and served on General Washington's staff for the remainder of the American Revolution. L'Enfant was promoted by brevet to Major in the Corps of Engineers on May 2, 1783, in recognition of his service to the cause of American liberty. He was discharged when the Continental Army was disbanded in December 1783.