Prince William County is located in Northern Virginia approximately 35 miles southwest of Washington, DC. Fairfax and Loudoun Counties bound the county on the north, on the east is the Potomac River (Maryland), on the south is the Stafford County and on the west is the Fauquier County.

Prince William County encompasses a total area of 348 square miles (222,615 acres) and a total land area of 338 square miles. Prince William County includes within its boundaries the independent cities of Manassas and Manassas Park. The combined area of Prince William County and the independent cities is 360 square miles (230,594 acres). Federal land accounts for approximately 41,500 acres or 18.6% of the total area and includes Quantico Marine Corps Base, Manassas National Battlefield Park and Prince William Forest Park.

Prince William County includes four incorporated towns and 14 census designated places, that is, unincorporated population centers that are designated by the U.S. Census Bureau for data collection purposes. Prince William County's highest elevation is 1,280 feet above sea level on Bull Run Mountain along its western boundary with Fauquier County. its lowest natural elevation is at sea level along the Potomac River, and its lowest man-made elevation is 15 feet below sea level at a quarry site near Manassas. The geographic center of the County is located near Brentsville at 38.55 degrees N. Latitude and 77.40 degrees W. Longitude.

Prince William county surrounding areas include Manassas city, Manassas Park city, Fairfax County, Stafford County, Fauquier County, and Loudoun County.