[3] The area that now comprises Yakima County was part of the Oregon Country at the start of the nineteenth century, inhabited both by fur prospectors from Canada, and Americans seeking land for agricultural and mineral-extraction opportunities.

During its existence, that provisional government formed the area north of the Columbia River first into the Washington Territory, and then (December 19, 1845) into two vast counties: Clark and Lewis.

However, several tribes felt the agreement creating this reserved area had been completed without sufficient native input, and skirmishes and local war (the Yakima War, which lasted until 1858) meant that the reservation was not fully operational for two decades.

Yakima County is reputed to be one of the most difficult places on earth to predict weather, because of its surrounding mountains.

The county's area is larger than the states of Delaware and Rhode Island combined.

[14] In terms of ethnicity, 15.8% reported German, 8.0% English, 7.3% Irish, and 3.6% American ancestry.

At the last census, the county harvested 1,027 acres (416 ha) of potato (Solanum tuberosum).

[18]: 395 The northeastern corner of the county is part of the U.S. Army's Yakima Training Center.

[19] The Yakima Valley AVA was established in 1983 in the state's oldest agricultural region.

It is Washington's third largest officially designated American Viticultural Area (AVA), and is responsible for more than 40% of the state's wine production.

[19] Common agricultural pests here include the Green Peach Aphid (Myzus persicae), the Spotted Cutworm (Xestia c-nigrum), and the Beet Leafhopper (Circulifer tenellus) in potato.

On only four occasions has it backed a Democrat for President – it has voted for the same candidate as the state of South Dakota in every election since they first voted for President – and the last Democratic gubernatorial candidate it backed was Albert D. Rosellini in 1956.

[22] Although Yakima County did narrowly support Maria Cantwell in 2012, the previous U.S. senate candidate to carry Yakima County for the Democrats was popular Senator "Scoop" Jackson in 1982.