[2] The county was founded in 1834 and named for Francis Marion, the American Revolutionary War hero known as the "Swamp Fox".

[citation needed]The first Catholic settlers in Kentucky came to Holy Cross in the western part of the county circa 1790.

According to planar projection maps of Kentucky, Marion County includes the center of the state of Kentucky located 3 miles NNW of Lebanon just off KY 429.

The actual physical center of Kentucky is disputed by surrounding counties[8] and due to use of planar methods[9] and projections used when the geographic centers of the United States were initially determined.

As of the census of 2000, there were 18,212 people, 6,613 households, and 4,754 families residing in the county.

24.40% of all households were made up of individuals, and 10.10% had someone living alone who was 65 years of age or older.