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.

When originally formed in 1838, Buchanan County was named Roberts County, after settler Hiram Roberts. It was renamed in 1839 for James Buchanan, then a U.S. Senator and later President of the United States. The county was formed from land annexed to Missouri, as were five other counties. The county seat is St. Joseph.

Carter County, located in the Ozarks region of southeast Missouri, was officially organized on March 10, 1859, from Oregon, Reynolds, Ripley and Shannon counties. It was named after Zimri A. Carter, a pioneer settler who came to Missouri from South Carolina in 1812.

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.

Holt County was organized January 29, 1841 from a territory of the Platte Purchase, located in the northwest corner of Missouri. The Missouri River forms a natural boundary to the south and west. It was originally named Nodaway County. The name changed seventeen days later to honor David Rice Holt, a Missouri Legislator, who died during the legislative session.

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.