Sullivan County is a county in the U.S. state of New York. As of the 2010 census, the population was 77,547. The county seat is Monticello. The county's name honors Major General John Sullivan, who was a hero in the American Revolutionary War. 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, New York's center of population was at the southern edge of Sullivan County.