Mill Run is a 9.1-mile-long (14.6 km)[1] tributary stream of the South Branch Potomac River, belonging to the Potomac River and Chesapeake Bay watersheds.[2] The stream is located in Hampshire County in the Eastern Panhandle of the U.S. state of West Virginia. Mill Run rises on Nathaniel Mountain and flows northeast, then northwest into the South Branch south of Romney near Hampshire Park on South Branch River Road (West Virginia Secondary Route 8).