The General Hugh Mercer building houses the Mercersburg Borough Hall and the Mercersburg Police Department. Located at 113 South Main Street in, Mercersburg, Franklin County, Pa. Originally called Blacktown, Mercersburg was incorporated in 1831. General Mercer was a General during the American Revolution and is, of course, the namesake of the town. The building, itself, was built in 1904 on the site of the first town jail.