There are 34 US Post Offices in Jefferson County serving a total of 106,767 residents equating to 3,140 residents per post office. It's estimated that approximately 170,412 packages are handled across the 34 county post offices annually. Jefferson County is a county located in the U.S. state of New York. As of the 2010 census, the population was 116,229. Its county seat is Watertown. The county is named after Thomas Jefferson, third President of the United States of America. It is adjacent to Lake Ontario, southeast from the Canada\u2013US border of Ontario. Jefferson County comprises the Watertown-Fort Drum, NY Metropolitan Statistical Area. The U.S. Tenth Mountain Division is based at Fort Drum."