There are 7 US Post Offices in Monroe County serving a total of 32,315 residents equating to 4,616 residents per post office. It's estimated that approximately 51,578 packages are handled across the 7 county post offices annually. Monroe County is a county located in the U.S. state of Illinois. According to the 2010 census, it had a population of 32,957. Its county seat and largest city is Waterloo. Monroe County is included in the St. Louis, MO-IL Metropolitan Statistical Area. It is located in the southern portion of Illinois known locally as \"Little Egypt\"."