[1] The county seat and largest city is Louisville.

[2] The county was created on February 20, 1796, and named for Thomas Jefferson, the main author of the Declaration of Independence who became the third president of the United States.

[4] The small northern portion of Jefferson County, defined by a line running from Stapleton southeast and just south of State Route 80, is located in the Brier Creek sub-basin of the Savannah River basin.

[5] As of the 2020 United States census, there were 15,709 people, 5,664 households, and 3,598 families residing in the county.

In 2024, Donald Trump became the first Republican since George H. W. Bush in 1988 to carry the county.