Around 1778, Duff was living in the Illinois Country, later referred to as the "American Bottom." While leading a group of longhunters returning to Kaskaskia, John Duff, John Saunders, and the rest of the hunting party were intercepted by Colonel George Rogers Clark's soldiers and his Virginia frontiersmen soldiers, near the ruins of Fort Massac. Suspected of being British spies, they immediately took an American oath of allegiance, where Duff and his men joined Clark's Illinois Regiment, Virginia State Forces. Duff enlisted into Captain John Williams' Company in Cahokia and rose to the rank of sergeant in the Illinois Regiment.