Green City is a city in northeast Sullivan County, Missouri, United States. The population was 657 at the 2010 census. The community is locateā¦Green City is a city in northeast Sullivan County, Missouri, United States. The population was 657 at the 2010 census. The community is located on Missouri Route 129 one half mile north of Missouri Route 6 and ten miles northeast of Milan. Yellow Creek flows past two miles to the west and the Union Ridge Conservation Area on Spring Creek is two miles to the northeast.