The county seat is Seattle, which is the state's largest city. About two-thirds of the county's population lives in the city's suburbs. King County ranks among the 100 highest-income counties in the United States.
As of the census of 2000, there were 1,737,034 people, 710,916 households, and 420,151 families residing in the county. The population density was 817 people per square mile (315/km²). There were 742,237 housing units at an average density of 349 per square mile (135/km²). The racial makeup of the county was 75.73% White, 5.40% Black or African American, 0.92% Native American, 10.81% Asian, 0.52% Pacific Islander, 2.56% from other races, and 4.06% from two or more races. 5.48% of the population were Hispanic or Latino of any race. 13.2% were of German, 9.1% English, 8.3% Irish and 5.5% Norwegian ancestry according to Census 2000. 81.7% spoke English, 4.2% Spanish, 2.3% Chinese (incl. all variations), 1.5% Vietnamese, 1.3% Tagalog and 1.0% Korean as their first language.
There were 710,916 households out of which 28.40% had children under the age of 18 living with them, 46.40% were married couples living together, 9.00% had a female householder with no husband present, and 40.90% were non-families. 30.50% of all households were made up of individuals and 7.50% had someone living alone who was 65 years of age or older. The average household size was 2.39 and the average family size was 3.03.
In the county, the population was spread out with 22.50% under the age of 18, 9.30% from 18 to 24, 34.70% from 25 to 44, 23.10% from 45 to 64, and 10.50% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 99.10 males. For every 100 females age 18 and over, there were 97.30 males.
The center of population of Washington in the year 2000 was located in the city of Enumclaw, in King County.
The median income for a household in the county was $53,157, and the median income for a family was $66,035 (these figures had risen to $64,915 and $82,879 respectively as of a 2007 estimate). Males had a median income of $45,802 versus $34,321 for females. The per capita income for the county was $29,521. About 5.30% of families and 8.40% of the population were below the poverty line, including 9.40% of those under age 18 and 7.40% of those age 65 or over.