There are 3 US Post Offices in Johnson County serving a total of 8,419 residents equating to 2,806 residents per post office. It's estimated that approximately 13,438 packages are handled across the 3 county post offices annually. Johnson County is a county located in the north central part of the U.S. state of Wyoming. As of the 2010 census, the population was 8,569. The county seat is Buffalo. Kaycee is the only other incorporated town in the county. Johnson County lies to the southeast of the Bighorn mountains along Interstate 25 and Interstate 90. The Powder River flows northward through eastern Johnson County."