Alexandria County was part of the original 100-mile square created as the District of Columbia in 1791 pursuant to Article I, Section 8, paragraph 17, of the United States Constitution. The portion of the District created from territory ceded by Virginia in Fairfax County was termed Alexandria County of the District of Columbia. It included all of the present Arlington County, Virginia, plus part of what is now the independent city of Alexandria, Virginia. The area was retroceded to Virginia by an act of the United States Congress on July 9, 1846 following a referendum of its citizens, and was then known as Alexandria County, Virginia.

1878 map of Alexandria County

Arlington County shares with a portion of the independent City of Alexandria (including the former town of Potomac) the distinction of being once in Virginia, then ceded to the US government to form the District of Columbia, and later retroceded to Virginia.

See also

• History of Washington, D.C.
• Arlington County, Virginia
• Retrocession (District of Columbia)
• Lost counties, cities and towns of Virginia
• List of extinct U.S. counties