Frederick County is located in the Commonwealth of Virginia. As of the 2020 census, the population was 91,419. Its county seat is Winchester. The county was formed in 1743 by the splitting of Orange County. It is Virginia's northernmost county. Frederick County is included in the Winchester, VA-WV Metropolitan Statistical Area, which is also included in the Washington-Baltimore-Northern Virginia, DC-MD-VA-WV-PA Combined Statistical Area.

A county seat is an administrative center, seat of government, or capital city of a county or civil parish. The term is in use in five countries: Canada, China, Hungary, Romania, and the United States. An equivalent term, shire town, is used in the U.S. state of Vermont and in several other English-speaking jurisdictions. County towns usually serve a similar function in Ireland, Jamaica, and the United Kingdom.