On Febuary 27, 1801, Alexandria County, which was land from the State of Virginia that was utilized to form part of the federal District of Columbia, was founded. However, local residents disagreed with the northern states views of abolition of slavery, hence in 1847, Arlington County retroceded from Washington, D.C. and became Alexandria County, Virginia.
In 1920, Alexandria County was renamed to Arlington County. A U.S. Supreme ruling dictated that Arlington County cannot be subdivided so currently the County Government also takes on municipal government functions.
The Arlington Historical Society has more information on the history of the county.