Seneca County is a county located in the U.S. state of New York. As of the 2010 census, the population was 35,251. The county seat is Waterloo. The name is from the Iroquois (Seneca) that occupied part of the region.
The area covered by Seneca County straddles the prehistoric territories of both Seneca and Cayuga Nations of the Iroquois League. When counties were established by Europeans in New York in 1683, the present Seneca County was part of Albany County. This was an enormous county, including the northern part of New York 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, and 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 now includes 37 counties of New York State. The county was named for William Tryon, colonial governor of New York.
In the years prior to 1776, most of the Loyalists in Tryon County fled to Canada. In the fall of 1779 on orders from commander-in-chief General George Washington the Sullivan Expedition conducted a scorched earth campaign against the Iroquois who sided with the Loyalists in the Revolutionary War. Sullivan's path destroyed Cayuga and Seneca villages along the east shore of Seneca Lake.
In 1784, following the peace treaty that ended the American Revolutionary War, the name of Tryon County was changed to Montgomery County in honor of the general, Richard Montgomery, who had captured several places in Canada and died attempting to capture the city of Quebec, replacing the name of the hated British governor.
In 1789, Montgomery County was reduced in size by the splitting off of Ontario County. The actual area split off from Montgomery County was much larger than the present county, also including the present Allegany, Cattaraugus, Chautauqua, Erie, Genesee, Livingston, Monroe, Niagara, Orleans, Steuben, Wyoming, Yates, and part of Schuyler and Wayne Counties.
Onondaga County was formed in 1794 by the splitting of Herkimer County.
Cayuga County was formed in 1799 by the splitting of Onondaga County. This county was, however, much larger than the present Cayuga County. It then included the present Seneca and Tompkins Counties and part of Wayne County.
In 1804, Seneca County was formed by the splitting of Cayuga County.
In 1817, Seneca County was reduced in size by combining portions of Seneca and the remainder of Cayuga County to form Tompkins County. Part of this territory, the current towns of Covert and Lodi, were returned to Seneca County in 1819.
In 1823, Seneca County was reduced in size by combining portions of Seneca and Ontario Counties to form Wayne County.