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The county was organized in November 1883 by the Washington Territorial Legislature, carved from the northern part of Yakima County.
Indigenous peoples known as Kittitas (or Upper Yakima) occupied the lands along the Yakima River for hundreds of years before the present era. The Kittitas Valley was a traditional gathering place for tribes east of the Cascades.
White settlers began pouring into the Kittitas Valley in the late 1850s. Their arrival forced dislocation and displacement of the native inhabitants, who were eventually forced into the Yakama Indian Reservation. White settlers introduced livestock raising, crop farming, dairying, logging, lumber processing, and mining. The abundant grassland and the generally-favorable terrain made beef and cattle production become the county's mainstay. That was assisted by the introduction of railways into the area and the large-scale irrigation systems introduced in the 1930s.
Wheat planting in Kittitas Valley began in 1868. The county's first flour mill was established near Ellensburg in 1873. Production of alfalfa was also seen from the county's early days.
Lumber extraction was an important county activity from its early days, mostly in the west end. Logging camps were established near the county's three largest lakes (Cle Elum, Kachess, Keechelus).
Mining for coal and minerals was established by the mid-1880s.
The southeastern corner of the county is part of the U.S. Army's Yakima Training Center.
There are numerous interpretations of the county's name, which is from the language of the Yakama Nation. According to one source, it "has been said to mean everything from 'white chalk' to 'shale rock' to 'shoal people' to 'land of plenty'". Most anthropologists and historians concede that each interpretation has some validity depending upon the particular dialect spoken.