Neosho is the most populous city in Newton County, Missouri, United States, which it serves as the county seat. With a population of 12,59…Neosho is the most populous city in Newton County, Missouri, United States, which it serves as the county seat. With a population of 12,590 as of the 2020 census, the city is a part of the Joplin, Missouri Metropolitan Statistical Area, a region with an estimated 176,849 residents. Neosho lies on the western edge of the Ozarks, in the far southwest of the state.