The County of Washington is one of the five political entities contained within the geographic region comprising what was originally the 100-square-mile District of Columbia. These were the City of Alexandria, the County of Alexandria , Georgetown, the City of Washington (Washington DC), and the County of Washington. Washington County was that area of the District that had been ceded by Maryland to the federal government and that excluded Georgetown and the City of Washington.

In an 1846 act, Congress returned the Virgina portions of the District to Virginia. In 1871, Congress enacted a territory act for the District by which "... all that part of the territory of the United States included within the limits of the District of Columbia be, and the same is hereby, created into a government by the name of the District of Columbia, by which name it is hereby constituted a body corporate for municipal purposes[.]" ("1871 Territory Act". The 1871 act merged the corporate charters of Georgetown and the City of Washington, and created a legislature for the entire District, with eleven members, including two for Georgetown and two for the County of Washington. This legislature was later superseded.)

In an 1895 act, Congress merged Georgetown into the City of Washington. ("An Act Changing the Name of Georgetown") (The status of the County of Washington was not addressed in this legislation, or subsequently, but there is no independent corporate government for the County, and the government of the City of Washington oversees the entire District.