There are 50 US Post Offices in Sullivan County serving a total of 74,934 residents equating to 1,499 residents per post office. It's estimated that approximately 119,603 packages are handled across the 50 county post offices annually. 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 county's 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. In 2010, the center of population of New York was located at the southern edge of Sullivan County."