Frederick County is a county located in Maryland, United States. As of the 2020 U.S. census, the population was 271,717. The county seat is Frederick. Frederick County is part of the Washington-Arlington-Alexandria, DC-VA-MD-WV Metropolitan Statistical Area. Like other outlying sections of the Washington metropolitan area, Frederick Cou…Frederick County is a county located in Maryland, United States. As of the 2020 U.S. census, the population was 271,717. The county seat is Frederick. Frederick County is part of the Washington-Arlington-Alexandria, DC-VA-MD-WV Metropolitan Statistical Area. Like other outlying sections of the Washington metropolitan area, Frederick County has experienced a rapid population increase since the 1980s. It borders the southern border of Pennsylvania and the northeastern border of Virginia.