Arlington County was originally named Alexandria County. It was formed from a portion of Fairfax County that Virginia in 1789 ceded to the federal government for use as the site of a new national capital. In 1801 the area officially became part of the District of Columbia, although Congress named it Alexandria County. By an act of 9 July 1846, Congress returned the county to Virginia, and the General Assembly extended the commonwealth's jurisdiction over the region effective 20 March 1847. By an act of assembly passed 16 March 1920, the county's name was changed to Arlington, the name of the Custis family mansion (the home of Robert E. Lee), which is located in the county. An urban county, Arlington contains no incorporated towns or cities. The county courthouse is in the county.

Alexandria, in Fairfax and Arlington Counties, was named for John Alexander, an early owner of the tract in Fairfax County on which the town was located. The act to establish Alexandria was passed in 1749. Its site had previously been known as Hunting Creek Warehouse and as Belhaven. Alexandria was incorporated as a town in 1779 and was ceded to the federal government in 1789 for use as part of the site of the new national capital. It officially became part of the District of Columbia in 1801 and was renamed Alexandria County by Congress. By an act of 9 July 1846, Alexandria County, including the town of Alexandria, was retroceded to Virginia, which took jurisdiction over the area on 20 March 1847. The town was incorporated as a city in 1852.