Andrew County, organized 1841, is one of six counties in the Indian Platte Purchase Territory annexed to Missouri in 1837. Named for Andrew Jackson Davis, a St. Louis editor, the county was first settled in the middle 1830s. Pioneers were from Ohio, Indiana, Tennessee, Kentucky, Virginia, and other parts of Missouri.

DeKalb County was organized on 25 February 1845 from Clinton County and is named after the American Revolutionary War general Johann de Kalb. Many records were lost in an 1878 courthouse fire.

Gentry County was organized February 14, 1841 and named for Colonel Richard Gentry of Boone County who fell in the Seminole War in 1837. The courthouse burned in 1885.

Organized January 26, 1833, from Cooper and Saline Counties, and named for Spencer Pettis, a Missouri congressman. Sedalia is the county seat and also home of the annual Missouri State Fair. Pettis is a strong rural county, but Sedalia was closely tied to the railroad lines passing through it. Those times are almost forgotten, except for the annual Ragtime Festival held to commemorate the partnership in Sedalia of music publisher John Stark and ragtime composer Scott Joplin.

Saline County was organized November 25, 1820, (effective January 1, 1821) from Cooper County and named for its numerous salt springs. Bordered on the north by the Missouri River, Saline County was considered to be part of "Little Dixie" in the antebellum years.

Taney County was officially organized on January 6, 1837 from a portion of Greene County and has since helped give birth to several more counties. Originally Taney covered a much larger area than we see today. Just 4 years after its formation, territory was taken to create Ozark County. In 1851 the western portion of the county was taken to form Stone County. In 1857 Douglas County was formed partly from Taney, and Howell County was taken from territory originally in Taney but now part of Ozark County. Then in 1859 the northern part was taken, along with territory from Greene and Webster Counties to form Christian County.

Worth County was organized February 8, 1861 and named for General William J. Worth who served in the Mexican-American War. It is the smallest county in the state in population and, excepting the independent city of St. Louis, the smallest in total area.