[3] The county was organized on December 23, 1826,[4][5] and named for General Francis Marion, the "Swamp Fox," who was from South Carolina and served in the American Revolutionary War.

The county seat is Palmyra, which was established in 1833 and became a locally important river port on the Mississippi.

Tobacco was the main crop, though cotton, wheat, barley, oats, and hay were also grown.

[8] The best known native son is Mark Twain (Samuel Langhorne Clemens) born in 1835, in the village of Florida.

His youthful experiences in Hannibal became the setting for the adventures of Tom Sawyer and Huckleberry Finn.

[9] Other prominent locals included Governor John S. Phelps, and Confederate General A.P.

The region was bitterly divided in the Civil War but Unionist elements prevailed.

It is home to a number of historic sites and buildings, with a major tourist industry focused on Mark Twain characters and settings.

Senator Hillary Clinton (D-New York) received more votes, a total of 1,587, than any candidate from either party in Marion County during the 2008 presidential primary.