There are 9 US Post Offices in Mason County serving a total of 16,745 residents equating to 1,861 residents per post office. It's estimated that approximately 26,727 packages are handled across the 9 county post offices annually. Mason County is a county located in the U.S. state of Illinois. According to the 2010 census, it had a population of 14,666. Its county seat is Havana. The county is named in honor of George Mason, a member of the Virginia legislature who campaigned for the adoption of the United States Bill of Rights."