All of the 476 square miles that are now Johnson County were once part of the Shawnee Indian reservation. The territory was opened to white settlement with the passing of the Kansas-Nebraska Act of 1854. At that time the area was crisscrossed by a number of major westward migration routes including the Ft. Leavenworth Military, the Santa Fe and Oregon-California trails.

Johnson County was created on August 25, 1855 by the Kansas Territorial Legislature. One of the first 33 counties in the state, it was named for the Reverend Thomas Johnson, founder of the Shawnee Methodist Mission.