There are 12 US Post Offices in Clinton County serving a total of 32,324 residents equating to 2,694 residents per post office. It's estimated that approximately 51,593 packages are handled across the 12 county post offices annually. Clinton County is a county located in the U.S. state of Illinois. As of the 2010 census, the population was 37,762. Its county seat is Carlyle. In 1960, the U.S. Census Bureau placed the mean center of U.S. population in Clinton County. Clinton County is part of the St. Louis, MO-IL Metropolitan Statistical Area.