Washington County is a county located in the western part of the U.S. state of Maryland. As of the 2010 U.S. Census, the population was 147,430. Its county seat is Hagerstown. Washington County was the first county in the United States to be named for the Revolutionary War general (and later President) George Washington.
Washington County is included in the Hagerstown-Martinsburg, MD-WV Metropolitan Statistical Area, which is also included in the Washington-Baltimore-Arlington, DC-MD-VA-WV-PA Combined Statistical Area. Washington County has experienced a population boom, and is one of the fastest growing counties in the country, due to an influx of people from Washington and Baltimore.
The western portions of Maryland (including present Washington County) were incorporated into Prince George's County in 1696. This original county included six current counties. The first to be created was Frederick, separated from Prince George's County in 1748.
Washington County was formed on September 6, 1776 by the division of Frederick County. At the same time, Montgomery County was also separated from Frederick County and was named for General Richard Montgomery. Washington County as created included land later to become Allegany County (created in 1789) and Garrett County (included in Allegany County when it was created in 1789, but separated from Allegany County in 1872). Washington County thus originally included the entire western part of the state.
A number of properties in the county are listed on the National Register of Historic Places.