South Mountain begins at the Potomac River as a low, narrow ridge, barely one mile wide and only 1,200 feet (370 m) above sea level at its crest.

South of the Potomac River in Virginia, the ridge continues as Short Hill Mountain for about 12 miles (19 km) before subsiding near the town of Hillsboro.

North of U.S. Route 30 in Pennsylvania, the South Mountain highlands reach their greatest width, over 12 miles (19 km), and several summits top 2,000 feet (610 m).

In a 1732 letter to the colonial governor of Maryland, Togotolisa, (Captain Civility), chief of the Susquehannock people, warns against settlement in the valley beyond the mountain.

[6] Following John Brown's Raid on Harpers Ferry in 1859, seven of the raiders escaped from their Kennedy Farm headquarters to Pennsylvania by following Elk ridge and South Mountain north.