Leavenworth County's population was estimated to be 73,628 in the year 2006, an increase of 4658, or +6.8%, over the previous six years; it has the fourth fastest growing and sixth largest population in the state.
As of the U.S. Census in 2000, there were 68,691 people, 23,071 households, and 17,210 families residing in the county. The population density was 148 people per square mile (57/km²). There were 24,401 housing units at an average density of 53 per square mile (20/km²). The racial makeup of the county was 84.18% White, 10.42% Black or African American, 0.74% Native American, 1.06% Asian, 0.13% Pacific Islander, 1.24% from other races, and 2.22% from two or more races. Hispanic or Latino of any race were 3.81% of the population.
There were 23,071 households out of which 38.90% had children under the age of 18 living with them, 61.40% were married couples living together, 9.50% had a female householder with no husband present, and 25.40% were non-families. 21.70% of all households were made up of individuals and 8.10% had someone living alone who was 65 years of age or older. The average household size was 2.69 and the average family size was 3.15.
In the county the population was spread out with 26.70% under the age of 18, 8.20% from 18 to 24, 33.00% from 25 to 44, 22.20% from 45 to 64, and 9.80% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 113.50 males. For every 100 females age 18 and over, there were 116.90 males.
The median income for a household in the county was $48,114, and the median income for a family was $55,805. Males had a median income of $40,047 versus $26,029 for females. The per capita income for the county was $20,292. About 4.80% of families and 6.70% of the population were below the poverty line, including 8.80% of those under age 18 and 7.50% of those age 65 or over.