Mason Neck is a peninsula jutting into the Potomac River in Fairfax County, Virginia, United States, southwest of Washington, D.C. It is surrounded by Belmont Bay to the west, the Potomac River to the south and east, Gunston Cove to the northeast, and Pohick Bay to the north-northeast. Mason Neck forms the southernmost section of Fairfax County in Northern Virginia. It comprises an area of 51.8 sqkm, two-thirds of which is preserved as parkland by regional, state, and national authorities. The population of the Mason Neck CDP was 2,005 as of the 2010 census.