Sullivan County is a county located in the U.S. state of New York. As of the 2010 census, the population was 77,547. The county seat is Monticello. The name is in honor of 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.