[2] The county was erected by the state of Ohio on February 20, 1820 and later reorganized in 1824.

[3] It is named for General Francis "Swamp Fox" Marion, a South Carolinian officer in the Revolutionary War.

25.10% of all households were made up of individuals, and 10.90% had someone living alone who was 65 years of age or older.

As of the 2010 United States Census, there were 66,501 people, 24,691 households, and 16,837 families living in the county.

[14] Prior to 1940, Marion County supported Democrats in presidential elections, only voting for Republican candidates five times from 1856 to 1936.

But starting with the 1940 election, the county has become a Republican stronghold in presidential elections with Lyndon B. Johnson being the only Democrat to win since, but Bill Clinton came within just 630 votes of winning it in 1996.

State Representative Tracy Richardson, elected in 2018, represents the 86th Ohio House district, and Riordan T. McClain, appointed in 2018, serves the 87th Ohio House District.

The entirety of Marion County lies within the 26th Ohio Senate District, currently represented by Bill Reineke, who defeated Democrat Craig Swartz in the 2020 Ohio Senate election.