Map - Seneca County, New York (Seneca County)

Seneca County is located in the U.S. state of New York. As of the 2020 census, the population was 33,814. The county seat is Waterloo. It became a one county in 1822, which currently remains in effect and uses one locations as county seats although the majority of Seneca County administrative offices are located in Waterloo. Therefore, most political sources list only Waterloo as the county seat. The county's name comes from the Seneca Nation of the Haudenosaunee (Iroquois), who occupied part of the region.

Seneca County comprises the Seneca Falls, NY Micropolitan Statistical Area, which is also included in the Rochester-Batavia-Seneca Falls, NY Combined Statistical Area.

The area covered by Seneca County straddles the prehistoric territories of both Seneca and Cayuga Nations of the Haudenosaunee. When counties were established by Europeans in New York in 1683, the present-day 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 Ontario County, also including the present Allegany, Cattaraugus, Chautauqua, Erie, Genesee, Livingston, Monroe, Niagara, Orleans, Steuben, Wyoming, Yates, and parts of Schuyler and Wayne counties.

Herkimer County was one of three counties split off from Montgomery County (the others being Otsego and Tioga counties) in 1791.

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.