At the time it was established, Yakima County was bounded by the Simcoe Mountains on the south, the Cascade Mountains on the west, the Wenatchee River on the north, and the Columbia River from below Wallula to Wenatchee on the east.
The Territorial Act establishing Yakima County named William J. Parker, J. H. Wilbur, and Charles Splawn county commissioners, William Wright county auditor, F. Mortimer Thorp county treasurer, and Gilbert Pell sheriff. The men appointed were all democrats and held these offices until 1868, when Yakima County held its first election.
William Wright's house was the first county seat, followed by Mortimer Thorp's house, C. P. Cooke's house, and finally a building in Yakima City donated for that purpose by O. D. and Sumner Barker, who as Barker Brothers operated the first store in Yakima City. Election results in 1870 on the question of where to place the county seat highlight the small number of settlers living in Yakima County at that time. The winner, Yakima City (also known as Mount Ottawa), received 89 votes; Flint's Store took 20 votes, Selah 18 votes, and the Kittitas Valley three votes.
Since the land included in Yakima County had not yet been surveyed, the settlers staked their own claims. In 1868 A. J. Splawn was deputized to assess property within Yakima County. Splawn later wrote in his memoir Ka-mi-akin: The Last Hero Of The Yakimas, "While assessing the property of Yakima County I had no disputes with the people. If they were poor, I passed them up; if well to do, they set their own valuation. We needed but little and wanted no surplus" (p. 160).
By the early 1880s the settlers in Ellensburg were agitating for either a county of their own, or to be named county seat of Yakima County. Despite opposition from some prominent residents of Yakima City, including Yakima Signal editor J. M. Adams, Kittitas County was carved out of Yakima County by Territorial Act on November 24, 1883.