Catoctin Mountain, along with the geologically associated Bull Run Mountains, forms the easternmost mountain ridge of the Blue Ridge Mountains, which are in turn a part of the Appalachian Mountains range. The ridge runs northeast–southwest for about 50 miles (80 km) departing from South Mountain near Emmitsburg, Maryland, and running south past Leesburg, Virginia, where it disappears into the Piedmont in a series of low-lying hills near New Baltimore, Virginia. The ridge forms the eastern rampart of the Loudoun and Middletown valleys.

South Mountain is the northern extension of the Blue Ridge Mountain range in Maryland and Pennsylvania. From the Potomac River near Knoxville, Maryland in the south to Dillsburg, Pennsylvania in York County, Pennsylvania in the north, the 70-mile-long (110 km) range separates the Hagerstown and Cumberland valleys from the Piedmont regions of the two states.