Spokane County was formed on January 29, 1858. It was annexed by Stevens County on January 19, 1864, and re-created on October 30, 1879.
As of the census of 2000, there were 417,939 people, 163,611 households, and 106,019 families residing in the county. The population density was 237 people per square mile (92/km²). There were 175,005 housing units at an average density of 99 per square mile (38/km²). The racial makeup of the county was 88.62% White, 2.00% Black or African American, 1.40% Native American, 1.88% Asian, 0.16% Pacific Islander, 0.82% from other races, and 2.76% from two or more races. 2.77% of the population were Hispanic or Latino of any race. 22.0% were of German, 10.7% Irish, 9.9% English, 7.6% United States or American and 6.4% Norwegian ancestry according to Census 2000.
There were 163,611 households out of which 32.40% had children under the age of 18 living with them, 49.90% were married couples living together, 11.00% had a female householder with no husband present, and 35.20% were non-families. 28.10% of all households were made up of individuals and 9.60% had someone living alone who was 65 years of age or older. The average household size was 2.46 and the average family size was 3.02.
In the county, the population was spread out with 25.70% under the age of 18, 10.60% from 18 to 24, 28.90% from 25 to 44, 22.40% from 45 to 64, and 12.40% who were 65 years of age or older. The median age was 35 years. For every 100 females there were 96.40 males. For every 100 females age 18 and over, there were 93.60 males.
The median income for a household in the county was $37,308, and the median income for a family was $46,463. Males had a median income of $35,097 versus $25,526 for females. The per capita income for the county was $19,233. About 8.30% of families and 12.30% of the population were below the poverty line, including 14.20% of those under age 18 and 8.10% of those age 65 or over.