Sullivan County is a county in the U.S. state of New York. As of the 2020 census, the population was 78,624. The county seat is Monticello. The county's name honors Major General John Sullivan, who was labeled at the time as a hero in the American Revolutionary War in part due to his successful campaign against the Iroquois (see Sullivan Expedition). The county was the site of hundreds of Borscht Belt hotels and resorts, which had their heyday from the 1920s through the 1970s. In 2010, the state's center of population was at the southern edge of Sullivan County.