There are 8 US Post Offices in Johnson County serving a total of 13,191 residents equating to 1,649 residents per post office. It's estimated that approximately 21,054 packages are handled across the 8 county post offices annually. Johnson County is a county located in the U.S. state of Illinois. According to the 2010 census, it has a population of 12,582. Its county seat is Vienna. It is located in the southern portion of Illinois known locally as \"Little Egypt\"."