There are 12 US Post Offices in Jefferson County serving a total of 62,481 residents equating to 5,207 residents per post office. It's estimated that approximately 99,727 packages are handled across the 12 county post offices annually. Jefferson County is a county located in the southern part of the U.S. state of Illinois. According to the 2010 census, it has a population of 38,827. The county seat is Mount Vernon. Jefferson County comprises the Mount Vernon, IL Micropolitan Statistical Area. It is located in the southern portion known locally as \"Little Egypt\". The western border of the county adjoins the Greater St. Louis consolidated metropolitan statistical area, the Metro-East region, and the St. Louis commuter region and market and viewing area."