Chantilly is a census-designated place in western Fairfax County, Virginia, United States. The population was 24,301 as of the 2020 census. Chantilly is named after an early-19th-century mansion and farm, which in turn took the name of an 18th-century plantation that was located in Westmoreland County, Virginia. The name "Chantilly" originated in France with the …Chantilly is a census-designated place in western Fairfax County, Virginia, United States. The population was 24,301 as of the 2020 census. Chantilly is named after an early-19th-century mansion and farm, which in turn took the name of an 18th-century plantation that was located in Westmoreland County, Virginia. The name "Chantilly" originated in France with the Château de Chantilly, about 28 miles north of Paris.