Mansfield is a city in Scott and Sebastian counties Arkansas, United States. The Sebastian County portion of the city is part of the Fort S…Mansfield is a city in Scott and Sebastian counties Arkansas, United States. The Sebastian County portion of the city is part of the Fort Smith, Arkansas-Oklahoma Metropolitan Statistical Area. As of the 2010 Census, the population was 1,139. The population was 1,097 at the 2000 census.