High Quality Content by WIKIPEDIA articles! Sullivan County is a county located in the U.S. state of New Hampshire. As of 2000, the population was 40,458. Its county seat is NewportSullivan County was organized at Newport in 1827, and is named for John Sullivan, revolutionary war hero and former governor. It was formed from the northern part of Cheshire County.According to the U.S. Census Bureau, the county has a total area of 552 square miles (1,430 km2), of which 537 sq mi (1,390 km2) is land and 15 square miles (39 km2) (2.63%) is water.