The County of Washington was one of five original political entities within the District of Columbia, the capital of the United States. Formed by the Organic Act of 1801 from parts of Montgomery and Prince George's County, Maryland, Washington County referred to all parts of the District of Columbia "on the east side of the Potomac, together with the islands therein." The bed of the Potomac River was considered to be part of Washington County as well. Alexandria County, containing the City of Alexandria formed the portion of the District ceded by the state of Virginia. It was returned to Virginia by Congress in 1846, thereby making Washington County and the District of Columbia coterminous.  At times, however, the term "Washington County" was used to refer to only the remainder of the District that was part of neither Georgetown nor Washington City. 
Starting in 1801, Washington County was governed by a board of commissioners, or a levy court, made up all of the Justices of the Peace, or magistrates, of the county appointed by the President and the number of those were not fixed.  In 1812, the board was reorganized with seven magistrates, two from east of Rock Creek but outside of Washington City, two from west of Rock Creek but outside of Georgetown and three from Georgetown, with none from Washington City until 1848 when four members from the City were added. The board was again changed 1863 when it was reduced to nine members, three from the city of Washington, one from Georgetown, and five from county lands outside the city.  These justices carried out the duties of county commissioners. Despite being within the federal territory, Congress left Washington County subject to the laws of Maryland. 
Washington County included Pleasant Plains, the estate of the Holmead family; Edgewood, home of Treasury Secretary Salmon P. Chase; and Petworth, the estate of Colonel John Tayloe III. Also contained in Washington County was the U.S. Soldiers' Retirement Home, where President Abraham Lincoln lived during his summers as President. Despite its comparatively large geographic size, Washington County was sparsely populated until the end of the 19th century.
Slavery was legal in Washington County, as it was in Maryland, but it was illegal to import a slave from Alexandria County for sale in Washington County. Slavery was ended in Washington County in April, 1862 by the District of Columbia Compensated Emancipation Act. It was the last part of the United States to end slavery before the 13th Amendment ended throughout the country.
During the American Civil War (1861–65), Washington County contained a partial circle of defensive fortifications that made Washington one of the most heavily fortified cities in the world at that time.  The forts surrounding Union-held territory in Virginia completed the defense circle. The Battle of Fort Stevens, July 11–12, 1864, took place in Washington County.
After the Civil War, many of the old estates in Washington County were sold and developed into suburbs for the growing capital city. Among the earliest developments were LeDroit Park and Mount Pleasant, which eventually became the first " streetcar suburb ". Uniontown and Barry Farm, a settlement for freedmen, developed east of the Anacostia River.
Washington County and the cities of Washington and Georgetown were abolished in 1871 following the passage of the District of Columbia Organic Act of 1871. This law brought the entire District of Columbia under the control of a territorial government headed by an appointed governor, an appointed eleven-member council, and a locally elected 22-member assembly. Two of the eleven council seats were reserved for representatives from the county outside of the cities of Washington and Georgetown.  Three years later, Congress abolished the territorial government in favor of direct rule over the District by an appointed three-member commission.