There are 3 US Post Offices in Campbell County serving a total of 20,792 residents equating to 6,931 residents per post office. It's estimated that approximately 33,186 packages are handled across the 3 county post offices annually. Campbell County is a county located in the U.S. state of Wyoming. As of the 2010 census, the population was 46,133, making it the third-most populous county in Wyoming. Its county seat is Gillette. Campbell County comprises the Gillette, WY Micropolitan Statistical Area."