There are 8 US Post Offices in Johnson County serving a total of 126,512 residents equating to 15,814 residents per post office. It's estimated that approximately 201,927 packages are handled across the 8 county post offices annually. Johnson County is a county located in the U.S. state of Indiana. As of 2010, the population was 139,654. The county seat is Franklin. Johnson County is included in the Indianapolis-Carmel-Anderson, IN Metropolitan Statistical Area."