Johnson County is a county in the U.S. state of Nebraska. As of the 2020 United States Census, the population was 5,290. Its county seat is Tecumseh. The county was formed in 1855, and was organized in 1857. It was named after Richard Mentor Johnson, who was Vice President of the United States from 1837 to 1841. In the Nebraska license plate system, Johnson County is represented by the prefix 57 (it had the fifty-seventh-largest number of vehicles registered in the county when the license plate system was established in 1922).