In 2008 CNN/Money and Money magazine ranked three cities in Johnson County on its list of the "100 Best Cities to Live in the United States." Overland Park was ranked ninth, and Olathe was ranked 11th, and Shawnee was ranked 39th. Olathe was also ranked 24th in the 2008 list of the top 25 fastest growing cities in the nation, compiled by the U.S. Census Bureau.
Johnson County's population was estimated to be 516,731 in the year 2006, an increase of 62,089, or +13.7%, over the previous six years; it has the fastest growing and largest population in the state.
As of the U.S. Census in 2000, there were 451,086 people, 174,570 households, and 121,675 families residing in the county. The population density was 946 people per square mile (365/km²). There were 181,612 housing units at an average density of 381 per square mile (147/km²). The racial makeup of the county was 91.11% White, 2.83% Asian, 2.61% Black or African American, 0.33% Native American, 0.03% Pacific Islander, 1.55% from other races, and 1.54% from two or more races. Hispanic or Latino of any race were 3.98% of the population. 25.1% were of German, 12.2% Irish, 12.0% English and 7.9% American ancestry according to Census 2000.
By 2005 85.8% of Johnson County's population was non-Hispanic whites. 3.7% of the population was African-American. An equal amount were of Asian descent. 0.4% of the population was Native American. 5.5% of the population was Latino.
There were 174,570 households out of which 36.00% had children under the age of 18 living with them, 59.20% were married couples living together, 7.80% had a female householder with no husband present, and 30.30% were non-families. 24.50% of all households were made up of individuals and 6.70% had someone living alone who was 65 years of age or older. The average household size was 2.56 and the average family size was 3.09.
In the county the population was spread out with 27.10% under the age of 18, 7.60% from 18 to 24, 32.80% from 25 to 44, 22.50% from 45 to 64, and 10.00% who were 65 years of age or older. The median age was 35 years. For every 100 females there were 95.50 males. For every 100 females age 18 and over, there were 92.00 males.
The median income for a household in the county was $61,455, and the median income for a family was $72,987. Males had a median income of $49,790 versus $32,145 for females. The per capita income for the county was $30,919. About 2.10% of families and 3.40% of the population were below the poverty line, including 3.30% of those under age 18 and 3.60% of those age 65 or over.
According to a 2007 Census Bureau estimate, the median income for a household and for a family had risen to $71,540 and $72,987 respectively.
Johnson County has a grid network through most of the county with a road every mile. The grid has facilitated rapid growth and easy access. Interstate 435 runs through much of the county, and serves as a developmental "border" in the along the northbound/southbound portion. The westbound/eastbound part of I-435 divides the county into a northern and southern section. The northern section is older, while the southern portion is the fastest-growing area in Johnson County, as well as containing a massive volume of new homes.
The Johnson County street grid begins at around Johnson Drive (near 55th street), and is a continuation of the adjacent Kansas City, Missouri street grid. The grid continues to about 175th street, with most suburban development ending around 159th street.
Other highways running through the area are Interstate 35, which runs diagonally through the county, starting from Downtown Kansas City, Missouri and continuing through Olathe, KS, Gardner, KS and eventually leads to the US-Mexico border, US Highway 69, which changes from Interstate 635 at the Wyandotte County border. 69 Highway eventually splits and goes on to form its own separate highway that runs southbound down halfway between the Johnson County-Kansas City, Missouri border at State Line Road and North/Southbound I-435.
Johnson County Transit is the public transit operator.