Catoctin Mountain, along with the geologically associated Bull Run Mountains, comprises 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 Aldie. The ridge forms the eastern rampart of the Loudoun and Middletown valleys.

Catoctin Mountain traverses Frederick County, Maryland, and extends into northern Loudoun County, Virginia. It rises to its greatest elevation of 1,900 feet (580 m) above sea level just southwest of Cunningham Falls State Park and is transected by gaps at Braddock Heights (Fairview Pass), Point of Rocks on the Potomac River and Clarke's Gap west of Leesburg, as well as several other unnamed passes in Maryland and Virginia. The mountain is much lower in elevation in Virginia, reaching its highest peak just south of the Potomac at Furnace Mountain (891 feet/271 m) and with only one peak above 800 feet (240 m) south of Leesburg.