Clay County is a county located in the U.S. state of Iowa. The population was 16,667 in the 2010 census, a decline from 17,372 in 2000. 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. Its county seat is Spencer. It also holds the annual Clay County Fair. The very first newspaper in Clay County was the Peterson Patriot which started circulation in 1880.