The county was established in 1879 when Idaho was a territory and named after U.S. President George Washington.

The racial makeup of the county was 87.61% White, 1.03% Asian, 0.66% Native American, 0.10% Black or African American, 0.07% Pacific Islander, 8.17% from other races, and 2.36% from two or more races.

23.5% of all households were made up of individuals, and 13.30% had someone living alone who was 65 years of age or older.

As of the 2010 United States census, there were 10,198 people, 4,034 households, and 2,803 families living in the county.

[11] In terms of ancestry, 20.0% were English, 18.2% were German, 14.8% were Irish, and 9.6% were American.

As of the 2020 United States census, there were 10,500 people, 4,035 households, and 2,708 families residing in the county.

[20] Washington County is in the area (but not the taxing region) of the College of Western Idaho, which has its main campus in Nampa.

[21] The county is served by the Southwest Idaho Health District.