As of the census of 2000, there were 78,153 people, 30,145 households, and 18,237 families residing in the county. The population density was 116 people per square mile (45/km²). There were 31,980 housing units at an average density of 47 per square mile (18/km²). The racial makeup of the county was 89.16% White, 0.84% Black or African American, 0.79% Native American, 4.49% Asian, 0.24% Pacific Islander, 1.92% from other races, and 2.56% from two or more races. 4.66% of the population were Hispanic or Latino of any race. 18.2% were of German, 11.6% English, 8.9% Irish and 7.0% American ancestry. 91.1% spoke English, 4.1% Spanish and 1.0% Chinese as their first language.
There were 30,145 households out of which 28.40% had children under the age of 18 living with them, 50.40% were married couples living together, 7.20% had a female householder with no husband present, and 39.50% were non-families. 26.10% 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 2.43 and the average family size was 2.95.
In the county, the population was spread out with 21.30% under the age of 18, 20.20% from 18 to 24, 26.70% from 25 to 44, 21.40% from 45 to 64, and 10.30% who were 65 years of age or older. The median age was 31 years. For every 100 females there were 99.10 males. For every 100 females age 18 and over, there were 97.80 males.
The median income for a household in the county was $41,897, and the median income for a family was $56,319. Males had a median income of $42,018 versus $29,795 for females. The per capita income for the county was $21,868. About 6.80% of families and 14.60% of the population were below the poverty line, including 10.60% of those under age 18 and 4.90% of those age 65 or over.
Benton County has the lowest church attendance per capita of any county in the nation (25% attendance).