In 1870, the population of Johnson County was 13,000.
In 1904, a man named W.B. Strang started constructing an interurban electric railroad between Kansas City and Olathe. It passed through Overland Park and Lenexa. A second one, called the Hocker Grove Line was built south and west of Merriam and Shawnee.
In the early 1910's, the northeastern part of the county was beginning to be developed. More and more famlies came toward Johnson County, lured by the many community developments such as Mission Hills and the Country Club District, designed by J.C. Nichols.
Between 1910 and 1940, the county's population went from 18,288 to 33,327. Most of this growth was due to the huge northeastern corner and the electric railroads. The population got even bigger after World War II. In 1950, it was 63,000. In the 60's, it almost doubled to 120,000. The biggest growth was still in the northeastern part of the county.
In the last half of the century, transportation corridors like Interstates 35 and 435 opened rural areas to new kinds of development. In the process, huge new employment patterns started to emerge. Between 1970 and 1984, the number of people employed by businesses in the county grew by over 200%.
By 2000, the population was 451,086; and now, it is about 560,000.