On March 25, 1800, Greene County was created by the partitioning of Albany (360 Square Miles) and Ulster (270 Square Miles) Counties, producing a county of 620 square miles (1,600 km).
On April 3, 1801, 90 square miles (230 km) of land was transferred from Delaware and Ulster Counties, raising the total area to 710 square miles (1,800 km). This transferred Prattsville, Vly Mountain, Halcott Center, Bushnellsville, Highmount, Shandaken, Lanesville, and Pine Hill within Greene County.
On May 26, 1812, Greene County lost 20 square miles (52 km) to Ulster County, lowering the total area to 690 square miles (1,800 km), 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, 30 square miles (78 km) of area was lost to Schoharie County, reducing the total area to 660 square miles (1,700 km), and reassigning Manorkill, producing the Greene County that exists today.