There are 8 US Post Offices in Clay County serving a total of 16,015 residents equating to 2,002 residents per post office. It's estimated that approximately 25,562 packages are handled across the 8 county post offices annually. Clay County is a county located in the U.S. state of Iowa. As of the 2010 census, the population was 16,667. Its county seat is Spencer. Its name is in honor of Henry Clay, Jr., a colonel who died in action in the Mexican-American War, and son of Henry Clay, famous American statesman. Clay County comprises the Spencer, IA Micropolitan Statistical Area. Clay County holds the annual Clay County Fair. The very first newspaper in Clay County was the Peterson Patriot which started circulation in 1880."