High Quality Content by WIKIPEDIA articles! Fairfax County is a county in Virginia, in the United States. Per the 2010 Census, the population of the county is 1,081,726, making it the most populous jurisdiction in the Commonwealth of Virginia, with 13.5% of Virginia`s population. The county is also the most populous jurisdiction in the Washington Metropolitan Area, with 19.8% of the MSA population, as well as the larger Baltimore–Washington Metropolitan Area, with 13.1% of the CSA population.