In 1854, the Kansas Territory was organized, then in 1861 Kansas became the 34th U.S. state. In 1855, Shawnee County was established.

Before the treaty of 1854, the area now known as Shawnee County was inhabited by Shawnee, Kansas, and Pottowatomie Indian tribes. Westward expansion brought the country its first white settler in 1830 when Frederick Choteau opened a trading post on American Chief (now Mission) Creek.  In 1855, Shawnee became one of the first counties established by the Kansas territorial legislature with a population of 250.  General H. J. Strickler, of Tecumseh, who was a member of the council in 1855, and also of the joint committee on Counties, claimed Shawnee for the name of his county.  At that time, Shawnee County borders were entirely south of the Kansas River and extended south to include Osage City and Carbondale.  The legislature later desired to make Topeka the county seat and moved the borders of the county to their present locations to make Topeka centrally located in the county.

1855 also saw the first ever meeting of the Shawnee County Board of Commissioners. Tecumseh was the first county seat, and the first county courthouse was opened there in 1856.  The building was 40x50 feet but was never finished. Topeka was made the county seat by popular vote in 1858, and a new courthouse was built at 4th Street and Kansas Avenue in 1867. In 1896, a new larger courthouse was constructed at 5th and Van Buren, with more than 50,000 residents then living in the county.  That building remained in use until the current courthouse at 7th and Quincy opened in 1965.