Ontario County was established in 1789 to govern lands the state of New York had acquired in the Phelps and Gorham Purchase; at the time it covered the entirety of Western New York. Steuben County, much larger than today, was split off from Ontario County on March 8, 1796. In 1823 a portion of Steuben County was combined with a portion of Ontario County to form Yates County. Steuben County was further reduced in size on April 17, 1854, when a portion was combined with portions of Chemung and Tompkins counties to form Schuyler County.
