[3] The county was formed in January 1839, and was named for U.S. President Thomas Jefferson.

[4][5] Jefferson County comprises the Fairfield, IA Micropolitan Statistical Area.

[14] As of the 2000 United States Census,[15] there were 16,181 people, 6,649 households, and 4,281 families in the county.

Jefferson County's executive branch is a three-member board of supervisors who are elected to four-year terms.

[18] Historically, Jefferson County heavily favored presidential candidates from the Republican Party.

From 1992 to 2012, the county had been carried by the Democratic presidential candidate in every election with the exception of 2000, when George W. Bush obtained a plurality.

In 2024, Donald Trump became the first Republican to win a majority of the votes in Jefferson County since Ronald Reagan's 1984 performance.