There are 3 US Post Offices in Hardin County serving a total of 3,902 residents equating to 1,301 residents per post office. It's estimated that approximately 6,228 packages are handled across the 3 county post offices annually. Hardin County is a county located in the U.S. state of Illinois. According to the 2010 census, it has a population of 4,320, making it the least populous county in Illinois. Its county seat is Elizabethtown. Hardin County is located in the part of the state known as Little Egypt."