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New York State County Formation Timeline for genealogy, is a timeline showing the formation of New York State counties, including dates and counties formed from.
| County | Formed From | Date | County Seat | Notes |
|---|---|---|---|---|
| Albany | Original County | 1683 | Albany | |
| Allegany | Genesee | 1806 | Belmont | |
| Bronx | New York | 1914 | Bronx | |
| Broome | Tioga | 1806 | Binghamton | |
| Cattaraugus | Genesee | 1808 | Genesee | |
| Cayuga | Onondaga | 1799 | Auburn | |
| Charlotte | Albany | 1772 | ||
| Chautauqua | Genesee | 1808 | Mayville | Renamed Washington in 1784 |
| Chemung | Tioga | 1836 | Elmira | |
| Chenango | Herkimer and tioga | 1798 | Norwich | |
| Clinton | Washington | 1788 | Plattsburg | |
| Columbia | Albany | 1786 | Hudson | |
| Cortland | Onondaga | 1808 | Cortland | |
| Delaware | Ulster, Otsego | 1797 | Delhi | |
| Dutchess | Original County | 1683 | Poughkeepsie | |
| Erie | Niagara | 1821 | Buffalo | |
| Essex | Clinton | 1799 | Elizabethtown | |
| Franklin | Clinton | 1808 | Malone | |
| Fulton | Montgomery | 1838 | Johnstown | |
| Genesee | Ontario | 1802 | Batavia | |
| Greene | Albany and Ulster | 1800 | Catskill | |
| Hamilton | Montgomery | 1816 | Lake Pleasant | |
| Herkimer | Montgomery | 1791 | Herkimer | |
| Jefferson | Oneida | 1805 | Watertown | |
| Kings | Original County | 1683 | Brooklyn | |
| Lewis | Oneida | 1805 | ||
| Livingston | Genesee, Ontario | 1821 | Geneseo | |
| Madison | Chenango | 1806 | Wampsville | |
| Monroe | Genesee, Ontario | 1821 | Rochester | |
| Montgomery | Albany | 1772 | Fonda | |
| Nassau | Queens | 1899 | Mineola | Until 1784 called Tryon |
| New York | Original County | 1683 | New York City | |
| Niagara | Genesee | 1808 | Lockport | |
| Oneida | Herkimer | 1798 | Utica | |
| Onondaga | Herkimer | 1794 | Syracuse | |
| Ontario | Montgomery | 1789 | Canandaigua | |
| Orange | Original County | 1683 | Goshen | |
| Orleans | Genesee | 1824 | Albion | |
| Oswego | Oneida, Onondaga | 1821 | Oswego | |
| Otsego | Montgomery | 1791 | Cooperstown | |
| Putnam | Dutchess | 1812 | Carmel | |
| Queens | Original County | 1683 | Jamaica | |
| Rensselaer | Albany | 1791 | Troy | |
| Richmond | Original County | 1683 | St. George | |
| Rockland | Orange | 1798 | New City | |
| St. Lawrence | Clinton, Herkimer, Montgomery | 1802 | Canton | |
| Saratoga | Albany | 1791 | Ballston Spa | |
| Schenectady | Albany | 1809 | Schenectady | |
| Schoharie | Albany and Otsego | 1795 | Schoharie | |
| Schuyler | Tompkins, Steuben, Chemung | 1854 | Watkins Glen | |
| Seneca | Cayuga | 1804 | Waterloo | |
| Steuben | Ontario | 1796 | Bath | |
| Suffolk | Original County | 1683 | Riverhead | |
| Sullivan | Ulster | 1809 | Monticello | |
| Tioga | Montgomery | 1791 | Owego | |
| Tompkins | Cayuga, Seneca | 1817 | Ithaca | |
| Tryon | Albany | 1772 | Renamed Montgomery in 1791 | |
| Ulster | Original County | 1683 | Kingston | |
| Warren | Washington | 1813 | Lake George | |
| Washington | Albany | 1772 | Hudson Falls | |
| Wayne | Ontario, Seneca | 1823 | Lyons | |
| Westchester | Original County | 1683 | White Plains | |
| Wyoming | Genesee | 1841 | Warsaw | |
| Yates | Ontario, Steuben | 1823 | Penn Yan |
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