The southwest face of Mount Nancy drains into Norcross Brook, thence into the East Branch of the Pemigewasset River, the Merrimack River, and into the Gulf of Maine at Newburyport, Massachusetts. The northwest side of Mt. Nancy drains into Anderson Brook, and thence into Norcross Brook. The northeast and southeast sides of Mt. Nancy drain into Nancy Brook, thence into the Saco River, and into the Gulf of Maine at Saco, Maine.