Greene County is a county located in the U.S. state of New York. As of the 2010 census, the population was 49,221. Its county seat is Catskill. The county's name is in honor of the American Revolutionary War general Nathanael Greene.
On April 3, 1801, of land was transferred from Delaware and Ulster counties, raising the total area to . This transferred Prattsville, Vly Mountain, Halcott Center, Bushnellsville, Highmount, Shandaken, Lanesville, and Pine Hill within Greene County.
On May 26, 1812, Greene County lost to Ulster County, lowering the total area to , reassigning Pine Hill, Highmount, and Shandaken to Ulster County.
On April 15, 1814, the county borders were re-surveyed, and the line adjusted with no change in area.
On April 17, 1822, the border was again surveyed, with no change in area, but Palenville was found to be inside Greene County.
On April 23, 1823, the border was again resurveyed, and the Hudson River border with Columbia County realigned accordingly.
On March 3, 1836, of area was lost to Schoharie County, reducing the total area to , and reassigning Manorkill, producing the Greene County that exists today.