Marion County is a county located in the northeastern portion of the U.S. state of Missouri. As of the 2010 census, the population was 28,781. Its county seat is Palmyra. Unique from most third class counties in the state Marion has two courthouses, the second located in Hannibal. The county was organized December 23, 1826 and named for General Francis Marion, the "Swamp Fox," who was from South Carolina and served in the American Revolutionary War.