When counties were established in New York State in 1683, the present Ontario County was part of Albany County. This was an enormous county, including the northern part of New York State as well as all of the present State of Vermont and, in theory, extending westward to the Pacific Ocean. This county was reduced in size on July 3, 1766 by the creation of Cumberland County, and further on March 16, 1770 by the creation of Gloucester County, both containing territory now in Vermont.
On March 12, 1772, what was left of Albany County was split into three parts, one remaining under the name Albany County. One of the other pieces, Tryon County, contained the western portion (and thus, since no western boundary was specified, theoretically still extended west to the Pacific). The eastern boundary of Tryon County was approximately five miles west of the present city of Schenectady, cutting through the Mohawk River valley. The county included the western part of the Adirondack Mountains and the area west of the West Branch of the Delaware River. The area then designated as Tryon County was later organized as 37 counties of New York State. The county was named for William Tryon, colonial governor of New York.
In the years shortly before 1776, most of the Loyalists in Tryon County fled to Canada, as tensions and physical conflicts were rising in central New York. In 1784, following the peace treaty that ended the American Revolutionary War, the name of Tryon County was changed to honor the Continental general, Richard Montgomery. He had captured several places in Canada and died trying to capture the city of Quebec. It replaced the name of the hated British governor. Seth Reed, a Colonel in the Battle of Bunker Hill, moved here with his family as a pioneer between 1787 and 1795. see also Geneva (town), New York
Land-hungry settlers from New England swept into upstate and western New York after the Revolution, as nearly five million acres of new lands were available for purchase since the Iroquois had been forced to cede most of their territories to the United States. Four tribes had allied with the British and were mostly resettled in Canada: the Mohawk, Onondaga, and Seneca.
In 1789, Ontario County was split off from Montgomery. The territory first organized as Ontario County was much larger than at present and lay along the shore of Lake Ontario. As the area was settled, new counties were organized. At first it included what are currently twelve other counties and parts of two more: Allegany, Cattaraugus, Chautauqua, Erie, Genesee, Livingston, Monroe, Niagara, Orleans, Steuben, Wyoming, and Yates counties, and parts of Schuyler and Wayne counties.
In 1796, Ontario County was divided and Steuben County was organized.
In 1802, Ontario County was reduced when Genesee County was split off. The new county was originally very large, including the present Allegany, Cattaraugus, Chautauqua, Erie, Niagara, Orleans and Wyoming Counties and parts of Livingston and Monroe counties.
In 1823, a portion of Seneca County was combined with a portion of Ontario County to create Wayne County. The same year, a portion of Steuben County was combined with a portion of Ontario County to create Yates County.