Washington County, Pennsylvania. Washington County is a county located in the U.S. state of Pennsylvania. As of the 2010 census, the population was 207,820. Its county seat is Washington. The county was created on March 28, 1781, from part of Westmoreland County. The city and county were both named after American Revolutionary War leader George Washington, who eventually became the first President of the United States. Washington County is part of the Pittsburgh, PA Metropolitan Statistical Area. The county is home to Washingto