[3] At the time of European colonization, the native tribes of the area that would become Prince William County were the Doeg, an Algonquian-speaking sub-group of the Powhatan tribal confederation.

When John Smith and other English explorers ventured to the upper Potomac River, beginning in 1608, they recorded the name of a village that the Doeg inhabited as Pemacocack (meaning "plenty of fish" in their language).

[6] The county was named for Prince William, Duke of Cumberland, the third son of King George II.

[7] The area encompassed by the 1731 act creating Prince William County originally included all of what later became the counties of Arlington, Fairfax, Fauquier, and Loudoun; and the independent cities of Alexandria, Fairfax, Falls Church, Manassas, and Manassas Park.

In the first two decades after the Revolution, the number and percentage of free blacks increased in Virginia as some whites freed their slaves, based on revolutionary ideals.

Into the early 20th century, the population was concentrated in two areas, one at Manassas (site of a major railroad junction), and the other near Occoquan and Woodbridge along the Potomac River, which was an important transportation route.

Continued suburbanization and growth of the Washington, D.C. metropolitan area caused that to increase rapidly in the following decades.

[14] Local resistance to the resort, because of its perceived adverse effects on the historic Manassas Battlefield, led to its end as a viable idea.

[17] The Marine Corps Heritage Museum and the Hylton Performing Arts Center opened in the 21st century.

[19] The Bureau of Economic Analysis combines the independent cities of Manassas and Manassas Park with Prince William County (within which the two cities are enclaves) for statistical purposes: The county is divided into seven magisterial districts: Brentsville, Coles, Potomac, Gainesville, Neabsco, Occoquan, and Woodbridge.

The board appoints a professional, nonpartisan county executive to manage operations of government agencies.

Republicans formerly held six of the eight Virginia House of Delegates seats that include parts of the county, with that delegation having consisted of Robert G. Marshall, Scott Lingamfelter, Tim Hugo, Jackson Miller, Rich Anderson, and Mark Dudenhefer.

Marshall, Lingamfelter, Miller, and Anderson all ran for reelection and were defeated by Democratic challengers Danica Roem, Elizabeth Guzmán, Lee Carter, and Hala Ayala respectively.

Dudenhefer opted to retire and instead successfully ran for a seat on the Stafford County Board of Supervisors, and he was replaced by Democrat Jennifer Carroll Foy.

Four of the five Virginia State Senate seats that include parts of the county are held by Democrats, including Democratic Sen. Jeremy Mc Pike, the President pro tempore of the Senate, Toddy Puller, George Barker and John Bell.

Democrats hold both of the U.S. Congressional seats that include parts of Prince William County.

In the 2008 United States presidential election, Democrat Barack Obama carried Prince William with 57.51% of the vote, compared to Republican John McCain who received 41.62%.

Obama's final rally the night before the election was held at the Prince William County Fairgrounds, just outside the city of Manassas.

Continuing demographic changes in the county, such as an increasingly diverse and urbanized population, were cited by The New York Times as contributing to Obama's success in the 2012 United States presidential election and suggesting the future appeal of the Democratic Party in the United States.

Prince William by 2012 had an educated professional population with the seventh-highest income in the country; it is the first county in Virginia to be composed of a majority of minorities: Hispanic, African American, and Asian.

The county has been a focal point for right-wing conspiracy theories about illegitimate votes during the 2020 presidential election.

However, the case was dropped,[23] and it was revealed that the errors in vote tabulation actually favored Trump, with no evidence of intentional fraud or significant impact on election outcomes.

According to the Census Bureau's 2009 American Community Survey,[38] 76.1% of the county's households are occupied by families, (compared to 66.5% in the United States).

According to the 2009 American Community Survey, the 2009 median household income in Prince William County was $89,785.

Opened in 1948, it was the location of the first commercial drag race held on the East Coast, and was a stop on the NASCAR Grand National schedule in the late 50s and early 60s.

The courthouse complex itself is located in a Prince William County enclave surrounded by the city of Manassas.