In 1608, Captain John Smith and his band of frontiersmen rode a barge along the Potomac River, the first white men to touch the unnamed wilderness that is now known as Prince William County. The county was formed in 1731 and was named for William Augustus, Duke of Cumberland, second son of King George II. The territory, which included Fairfax, Arlington, Alexandria, Loudon and Fauquier, was reduced to its present size in 1759.

In 1861, the sleepy railroad community of Manassas Junction suddenly became one of the most important places in American history. Before the outbreak of the war, the town was made up of just four buildings. It was better known by the name of its post office, Tudor Hall. Both the history of the United States and the history of the Northern Virginia Piedmont were shaped by the events that occurred in Prince William County. In addition to the stretches of battleground now preserved in the Manassas National Battlefield Park, Prince William County is home to several other important sites in the Civil War that illustrate the crucial role this area played during this episode in American history.

With the construction of the county courthouse in 1822, Brentsville became the county seat. Long before, it was the crossroads of Indian trade paths and roads from the Potomac to the Blue Ridge Mountains. It remained the county seat until 1892, when it was moved to Manassas.