Benton County was created out of Klickitat County on March 8, 1905.
As of the census of 2000, there were 142,475 people, 52,866 households, and 38,063 families residing in the county. The population density was 84 people per square mile (32/km²). There were 55,963 housing units at an average density of 33 per square mile (13/km²). The racial makeup of the county was 86.25% White, 0.93% Black or African American, 0.82% Native American, 2.20% Asian, 0.11% Pacific Islander, 7.01% from other races, and 2.69% from two or more races. 12.50% of the population were Hispanic or Latino of any race. 18.1% were of German, 11.0% English, 9.1% United States or American and 8.4% Irish ancestry according to Census 2000. 86.4% spoke English and 10.3% Spanish as their first language.
There were 52,866 households out of which 38.20% had children under the age of 18 living with them, 57.60% were married couples living together, 10.20% had a female householder with no husband present, and 28.00% were non-families. 23.20% of all households were made up of individuals and 7.70% had someone living alone who was 65 years of age or older. The average household size was 2.68 and the average family size was 3.17.
In the county, the population was spread out with 29.70% under the age of 18, 8.60% from 18 to 24, 28.50% from 25 to 44, 22.90% from 45 to 64, and 10.30% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 98.70 males. For every 100 females age 18 and over, there were 96.30 males.
The median income for a household in the county was $47,044, and the median income for a family was $54,146. Males had a median income of $45,556 versus $27,232 for females. The per capita income for the county was $21,301. About 7.80% of families and 10.30% of the population were below the poverty line, including 14.30% of those under age 18 and 6.90% of those age 65 or over.