The region of southern Rhode Island that encompasses Washington County today was the subject of conflicting land claims by the late 1650s. The English colonies of Massachusetts Bay, Connecticut and Rhode Island as well as two different land companies claimed jurisdiction and title. In 1664 a royal commission under Charles II stepped in to adjudicate these conflicting land claims over the so-called Narragansett Country. The commission extinguished the claims of Massachusetts, and Rhode Island was granted jusrisdiction until the commission finished processing Connecticut's appeals, which weren't ended until 1726. Settlements of King's Province were named to reflect the English Restoration (like Charleston County, South Carolina), because King Charles II of England also finalized Rhode Island's charter.
As of the census of 2000, there were 123,546 people, 46,907 households, and 32,037 families residing in the county. The population density was 371 people per square mile (143/km²). There were 56,816 housing units at an average density of 171 per square mile (66/km²). The racial makeup of the county was 94.82% White, 0.92% Black or African American, 0.93% Native American, 1.50% Asian, 0.02% Pacific Islander, 0.46% from other races, and 1.35% from two or more races. 1.44% of the population were Hispanic or Latino of any race. 19.2% were of Italian, 18.5% Irish, 14.7% English, 6.7% French and 5.8% German ancestry according to Census 2000. 92.8% spoke English, 1.8% Spanish and 1.6% Italian as their first language.
There were 46,907 households out of which 31.80% had children under the age of 18 living with them, 55.50% were married couples living together, 9.40% had a female householder with no husband present, and 31.70% were non-families. 24.10% of all households were made up of individuals and 9.20% had someone living alone who was 65 years of age or older. The average household size was 2.52 and the average family size was 3.01.
In the county, the population was spread out with 23.40% under the age of 18, 11.20% from 18 to 24, 28.30% from 25 to 44, 24.40% from 45 to 64, and 12.80% who were 65 years of age or older. The median age was 37 years. For every 100 females there were 94.20 males. For every 100 females age 18 and over, there were 91.30 males.
The median income for a household in the county was $53,103, and the median income for a family was $64,112. Males had a median income of $43,956 versus $30,659 for females. The per capita income for the county was $25,530. About 4.20% of families and 7.30% of the population were below the poverty line, including 7.40% of those under age 18 and 5.60% of those age 65 or over.