There are 3 US Post Offices in Brown County serving a total of 6,902 residents equating to 2,301 residents per post office. It's estimated that approximately 11,016 packages are handled across the 3 county post offices annually. Brown County is a county located in the U.S. state of Illinois. As of the 2010 census, the population was 6,937. Its county seat is Mount Sterling. Siloam Springs State Park is located partly in this county."