Mercersburg is a borough in Franklin County, Pennsylvania, United States. It is 72 miles (116 km) southwest of Harrisburg, the state capital. Originally called "Black Town", it was incorporated in 1831. Locals also used to refer an area outside of the town as "Little Africa" due to the large population of African Americans who lived here during the time of the Underground Railroad. However as of 2010 only 6.49 percent of the population is African American. The population was 1,561 at the 2010 census.
Mercersburg was platted in 1780. The borough was named after Hugh Mercer, a general in the American Revolutionary War. A post office has been in operation at Mercersburg since 1812.
The Mercersburg Historic District, Lane House, and Mercersburg Academy are listed on the National Register of Historic Places.
Mercersburg is located in southwestern Franklin County at 39°49′46″N 77°54′7″W (39.829388, -77.901952). According to the United States Census Bureau, the borough has a total area of 0.89 square miles (2.3 km2), all of it land.
Mercersburg is located at the eastern foot of Cove Mountain on the west side of the Great Appalachian Valley. The borough is 7.5 miles (12.1 km) north of the Mason–Dixon line, which serves as the state border with Maryland.