In Tennessee's Ordinance of Secession referendum on June 8, 1861, Decatur County voted to remain in the Union by a margin of 550 to 310,[4] being one of only eight counties in West or Middle Tennessee to support the Union.

Earlier on February 9, 1861, Decatur County voters had voted against holding a secession convention by a margin of 514 to 251.

[5] In 2015, the Decatur County clerk of court and the entire staff of that office resigned, to express conscientious objection to the United States Supreme Court's ruling in Obergefell v. Hodges, which would oblige the office to issue marriage licenses to same-sex couples.

[7] As of the 2020 United States census, there were 11,435 people, 4,440 households, and 3,059 families residing in the county.

The last Democrat to carry this county was Al Gore in 2000, and even before the collapse of traditional rural Democratic support after Bill Clinton the county had a sizeable Unionist population that caused it to vote Republican several times during the “System of 1896”.