Kibby Mountain stands within the watershed of the Kennebec River, which drains into the Gulf of Maine. The north side of Kibby Mountain drains into the East Branch of the Moose River, then into the South Branch of the Moose, the Moose River, and the Kennebec River. The west side of Kibby Mountain drains into Caribou Flow, then into the South Branch of the Moose River. The southwest end of Kibby Mountain drains into the Middle Branch of Kibby Stream, then into Spencer Stream and the Dead River, another tributary of the Kennebec. The southeast side of Kibby Mountain drains into the West Branch of Spencer Stream.