There are 9 US Post Offices in Warren County serving a total of 19,314 residents equating to 2,146 residents per post office. It's estimated that approximately 30,827 packages are handled across the 9 county post offices annually. Warren County is a county located in the U.S. state of Illinois. According to the 2010 census, it had a population of 17,707. Its county seat is Monmouth."