There are 10 US Post Offices in Jefferson County serving a total of 75,521 residents equating to 7,552 residents per post office. It's estimated that approximately 120,540 packages are handled across the 10 county post offices annually. Jefferson County is a county located in the U.S. state of Wisconsin. As of the 2010 census, the population was 83,686. Its county seat is Jefferson. Jefferson County comprises the Watertown-Fort Atkinson, WI Micropolitan Statistical Area, which is also included in the Milwaukee-Racine-Waukesha, WI Combined Statistical Area."