There are 3 US Post Offices in Marion County serving a total of 10,834 residents equating to 3,611 residents per post office. It's estimated that approximately 17,292 packages are handled across the 3 county post offices annually. Marion County is a county located in the U.S. state of Texas. As of the 2010 census, its population was 10,546. Its county seat is Jefferson. Marion County is in East Texas and is named for Francis Marion, the Revolutionary War general from South Carolina who was nicknamed the \"Swamp Fox\".