The British Headquarters Map, circa 1782, provides a remarkable window onto the natural topography, hydrology, and land cover of Manhattan Island, NY, before extensive urbanization. Manhattan formerly hosted a rugged topography watered by over 108 km of streams and at least 21 ponds, flowing in and out of wetlands that covered nearly 10% of the island in the late 18^th century. These features are largely representative of the landscape prior to European settlement. We used ecological features interpreted from the British Headquarters Map, and additional historical, ecological, and archeological information, to hypothesize about the ecosystem composition of the pre-European island. We suggest that 54 different ecological communities may have once been found on the island or in nearby waters, including chestnut-tulip tree forests, Hempstead Plains grasslands, freshwater and tidal marshes, hardwood swamps, peatlands, rocky headwater streams, coastal-plain ponds, eelgrass meadows, and culturally derived ecosystems, such as Native American village sites and fields. This former ecosystem mosaic, consisting of over 99% natural areas, stands in sharp contrast to the 21^st-century state of the island in which only 3% of its area is dedicated to ecological management.