Mount Vernon is a census-designated place (CDP) in Fairfax County, Virginia, United States. The population was 28,582 at the 2000 census.

While "Mount Vernon" -- drawn from the Mount Vernon plantation, the home of George Washington located south of Alexandria -- is often used locally to refer to the entire unincorporated area between Old Town Alexandria and Fort Belvoir, Mount Vernon as defined by the Census Bureau encompasses only the part of it coextensive with Alexandria ZIP code 22309, bounded by the Potomac River to the south, Fort Belvoir to the west, Huntley Meadows Park to the north, and Little Hunting Creek to the east. It covers a wide socioeconomic swath, from low-to-moderate-income trailer parks and apartments along U.S. Route 1 to million-dollar-plus homes near the river just 1-2 miles away. The Mount Vernon estate itself is located here, and while it has its own post office (ZIP 22121), there are no delivery routes from it to the surrounding area.