The U.S. Census Bureau has defined Harrison Township as a census-designated place (CDP) in the 2000 Census. This allows the community to appear both on the list of places (like cities and villages) as well on the list of county subdivisions (like other townships). The final statistics for the township and the CDP are identical.
As of the census of 2000, there were 24,461 people, 10,720 households, and 6,421 families residing in the township. The population density was 1,710.2 per square mile (660.5/km²). There were 11,486 housing units at an average density of 803.0/sq mi (310.1/km²). The racial makeup of the township was 94.53% White, 2.47% African American, 0.38% Native American, 0.58% Asian, 0.04% Pacific Islander, 0.45% from other races, and 1.55% from two or more races. Hispanic or Latino of any race were 1.48% of the population.
There were 10,720 households out of which 26.9% had children under the age of 18 living with them, 47.5% were married couples living together, 8.1% had a female householder with no husband present, and 40.1% were non-families. 33.1% of all households were made up of individuals and 8.2% had someone living alone who was 65 years of age or older. The average household size was 2.28 and the average family size was 2.94.
In the township the population was spread out with 21.9% under the age of 18, 8.7% from 18 to 24, 32.5% from 25 to 44, 26.0% from 45 to 64, and 10.9% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 101.2 males. For every 100 females age 18 and over, there were 100.7 males.
The median income for a household in the township was $51,892, and the median income for a family was $67,129. Males had a median income of $47,444 versus $31,561 for females. The per capita income for the township was $29,491. About 3.6% of families and 5.7% of the population were below the poverty line, including 7.0% of those under age 18 and 5.9% of those age 65 or over.