There are 9 US Post Offices in Marion County serving a total of 32,492 residents equating to 3,610 residents per post office. It's estimated that approximately 51,861 packages are handled across the 9 county post offices annually. Marion County is a county located in the U.S. state of Iowa. As of the 2010 census, the population was 33,309. The county seat is Knoxville. It is named for Francis Marion, a brigadier general from South Carolina in the American Revolutionary War."