It was named in honor of Revolutionary War Gen. Francis Marion, the "Swamp Fox".

[4] The southwest corner of Marion County is the intersection of the Baseline with the Third Principal Meridian, the point of origin for the third survey of the Northwest Territory under the Land Ordinance of 1785.

[5] As of the 2010 United States census, there were 39,437 people, 16,148 households, and 10,746 families residing in the county.

Between 1912 and 2004 it was a perfect bellwether apart from the Catholicism-influenced 1960 election when substantial anti-Catholic voting by its largely southern white population caused it to support Republican Richard Nixon.

Since the beginning of the twenty-first century it has voted consistently for Republican presidential candidates.