Established in 1789, Ontario County encompassed all of western New York State from the Pre-emption Line to Lake Erie and from Lake Ontario to the Pennsylvania border. Now fourteen separate counties, the early history of this vast area is retained in the records of the parent County.
The Hartford Treaty of 1786 settled the conflicting claims of Massachusetts and New York, awarding New York political sovereignty and Massachusetts the right of pre-empting and selling the Iroquois-controlled land. In 1787, Massachusetts sold its rights to the six million acre tract to Oliver Phelps and Nathaniel Gorham. The Buffalo Creek Treaty, negotiated by Phelps, eliminated the native American claim to the 2,250,000-acre eastern portion. The western part reverted to Massachusetts ownership and was later sold to Pennsylvania financier Robert Morris.
Phelps and Gorham surveyed the Purchase into ranges and townships, establishing the first land office of its kind in Canandaigua. Settlers from eastern New York and New England poured into the county, attracted by the fertile soil, temperate climate, and low prices. As the population increased, the New York State Legislature was pressured to subdivide and form new counties. Thus Steuben was taken off in 1796, Genesee in 1802, Livingston and Monroe in 1821, and Wayne and Yates in 1823.
The history of the Ontario County pioneers may be found today in the holdings of the Ontario County Records and Archives Center. Assessment records for Geneseo in 1800 and Boyle in 1813, the 1789 deed to Mary Jemison from the Seneca Nation, and the 1809 petition for dowry restoration by widow Triphena Stewart of Bloomfield, are just some examples of the early records available for research.