There are 20 US Post Offices in Grant County serving a total of 50,709 residents equating to 2,535 residents per post office. It's estimated that approximately 80,937 packages are handled across the 20 county post offices annually. Grant County is a county located in the U.S. state of Wisconsin. As of the 2010 census, the population was 51,208. Its county seat is Lancaster. Grant County comprises the Platteville, WI Micropolitan Statistical Area. It is in the tri-state area of Illinois, Iowa, and Wisconsin, and is crossed by travelers commuting to Madison from a number of eastern Iowan cities, and by residents of northern Illinois traveling to the Twin Cities or La Crosse, Wisconsin.