Union City is a city in Fulton County, Georgia, United States. The population was 26,830 at the 2020 census. The Georgia General Ass…Union City is a city in Fulton County, Georgia, United States. The population was 26,830 at the 2020 census. The Georgia General Assembly incorporated Union City in 1908. One source claims that the town was named for the union of rails at a railroad junction, while another version states the name commemorates a "Farmer's Union" which once was headquartered here.