For many millennia, the Great Plains of North America was inhabited by nomadic Native Americans.

In 1802, Spain returned most of the land to France, but keeping title to about 7,500 square miles.

In 1803, most of the land for modern day Kansas was acquired by the United States from France as part of the 828,000 square mile Louisiana Purchase for 2.83 cents per acre.

[4] Settlement of the county was slowed by events prior to and during the Civil War, but the present day unincorporated community of Thompsonville (3 miles northwest of Perry on the Delaware River) was the first established in 1851 by Mormon settlers who initially refused to accompany the main group led by Brigham Young to the Salt Lake Valley.

[5] Jefferson County is included in the Topeka, KS Metropolitan Statistical Area.

Jefferson County was a prohibition, or "dry", county until the Kansas Constitution was amended in 1986 and voters approved the sale of alcoholic liquor by the individual drink with a 30% food sales requirement.

[13] Jefferson County has two newspapers which are still in operation, The Oskaloosa Independent and The Valley Falls Vindicator.

[16] † means a community is designated a Census-Designated Place (CDP) by the United States Census Bureau.