The mountain is located within the towns of Mount Washington, Massachusetts and Salisbury, Connecticut; much of it has been conserved as part of Massachusetts' Mount Washington State Forest and Connecticut's Riga Mountain Forest Preview. The south side of Mount Frissell drains into Riga Lake and South Pond, then into Wachocostinook Brook, Salmon Creek, the Housatonic River, and Long Island Sound. The northwest side drains into Ashley Hill Brook, thence Bash Bish Brook, the Roeliff Jansen Kill, the Hudson River, and Upper New York Bay. The northeast side drains into Sages Ravine, thence into Schenob Brook, the Hubbard Brook, the Housatonic River, and Long Island Sound. Mount Frissell is bordered by Round Mountain to the southeast, Mount Ashley to the north, and Brace Mountain to the west.