The city (population 35,135 per Census 2000) that arose from the village of Manassas Junction after the American Civil War. The site of the city incorporates the railroad junction that gave it its name (the name itself comes from the Manassas Gap Railroad, the line leading northwest to Winchester, VA, which is now part of the Norfolk Southern system), and sits about 3 miles away from Manassas National Battlefield Park. The battles of First Manassas and Second Manassas were fought in the immediate vicinity; it's now an important suburb of Washington, D.C., which is about 30 miles away.

It's surrounded on all but one side by Prince William County, Virginia; the city of Manassas Park, VA resides just north of the city, and shares a border with it in that area. It's Prince William's county seat, and they share the courthouse there. A few County offices (and one each of their high schools and middle schools) are still within their borders, though most County operations have moved to locations further away.

The city was first incorporated as a town in 1873; the city gained its independence from the County, along with Manassas Park, in 1975, under Virginia's Independent City rules.