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, 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 Virginia 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^*" (The 1871 act merged the corporate charters of Georgetown and the City of Washington and brought the entire District of Columbia together under a single eleven-member legislature, including two representatives for Georgetown and two for the County of Washington.)

Washington County ceased to exist in 1878, when Congress passed the DC Organic Act that merged all of the District of Columbia into the City of Washington. (Georgetown remained nominally separate until 1895.)

See also

Defunct counties of the United States | History of the District of Columbia