In 1814, Andrew Moore departed with his eight-year-old son for Jordan's settlement, a journey from which he never returned.

In 1816, Carter Wilkey, Daniel Crenshaw and Robert Cook settled in Moore's Prairie.

It was named in honor of Thomas Jefferson, principal draftsman of the Northwest Ordinance, among other things.

[5] As of the 2010 United States census, there were 38,827 people, 15,365 households, and 10,140 families residing in the county.

Jefferson is politically a fairly typical “anti-Yankee” Southern Illinois county.

It was to again vote Republican in the greater landslides of 1920 and 1928, but otherwise was firmly Democratic until World War II.

Following the New Deal, Jefferson became something of a bellwether county, voting for every winning presidential candidate between 1928 and 2004 except in the Catholicism-influenced 1960 election, and that of 1988 which was heavily influenced by a major Midwestern drought.

Disagreement with the Democratic Party's liberal views on social issues since the 1990s has caused a powerful swing to the GOP in the past quarter-century:[15] as is typical of the Upland South, Barack Obama in 2012 and Hillary Clinton did far worse than any previous Democrat.

Jefferson County is divided into sixteen townships: School districts include:[17]