[4] The CDP consists of several suburban neighborhoods in the Harrisburg area, including White Hill, Highland Park, Eberlys Mill, Cedar Cliff, and Allendale.

Yellow Breeches Creek forms part of the southern boundary of the CDP, across which is Fairview Township in York County.

Interstate 83 runs along the eastern edge of Lower Allen, with access from Exits 40B and 41A.

The Harrisburg Capital Beltway (Pennsylvania Route 581) runs along the northern edge of the CDP.

U.S. Route 15 forms the western extent of the CDP, with the nearest access from Lower Allen Drive at the Capital City Mall, just outside the CDP's limit.According to the U.S. Census Bureau, Lower Allen has a total area of 2.2 square miles (5.8 km2), of which 2.2 square miles (5.7 km2) is land and 0.04 square miles (0.1 km2), or 1.51%, is water.