Mercersburg is a borough in Franklin County, Pennsylvania, 73 miles (117 km) southwest of Harrisburg. Originally called Black Town, it was incorporated in 1831. In 1900, 956 people lived here, and in 1910, 1,410 people lived here. The population was 1,540 at the 2000 census.
Mercersburg is located at 39°49′46″N 77°54′7″W / 39.82944°N 77.90194°W / 39.82944; -77.90194 (39.829388, -77.901952).
According to the United States Census Bureau, the borough has a total area of 1.0 square miles (2.5 km²), all of it land.