Marion county was formed in 1834. It is located in the Western Knobs region of the state. The elevation in the county ranges from 475 to 1260 feet above sea level. In 2010 the county population was 19,820 in a land area of 343.01 square miles, an average of 57.8 people per square mile.

The county seat is Lebanon.

The geographic center of Kentucky is in Marion county, just northwest of Lebanon.