There are 3 US Post Offices in Marion County serving a total of 25,206 residents equating to 8,402 residents per post office. It's estimated that approximately 40,232 packages are handled across the 3 county post offices annually. Marion County is a county located in the U.S. state of Mississippi. As of the 2010 census, the population was 27,088. Its county seat is Columbia. Marion County is named for American Revolutionary War guerrilla leader Francis Marion."