There are 11 US Post Offices in Marion County serving a total of 46,648 residents equating to 4,241 residents per post office. It's estimated that approximately 74,455 packages are handled across the 11 county post offices annually. Marion County is a county located in the U.S. state of Illinois. According to the 2010 census, it had a population of 39,437. Its county seat is Salem. Marion County comprises the Centralia, IL Micropolitan Statistical Area, which is included in the St. Louis-St. Charles-Farmington, MO-IL Combined Statistical Area."