Montclair is a census-designated place (CDP) in Prince William County, Virginia, United States. The population was 19,570 at the 2010 census. Montclair is a residential community surrounding a man-made lake and golf course. Development began in the late 1960s and new home construction for the most part ended in the 1990s. Traditionally, residents of Montclair used the mailing address and ZIP code of Dumfries, although the town of Dumfries is several miles from Montclair.