South Twin forms the high point of a north–south ridge, with North Twin Mountain lying approximately one mile to the north and Mount Guyot two miles (3.2 km) to the southeast.

Galehead Mountain, a lower spur of South Twin, is to the west.

Southwest of South Twin Mountain is the valley of Twin Brook, which flows into the Franconia Branch of the East Branch of the Pemigewasset River, through the Pemigewasset Wilderness, thence into the Pemigewasset and Merrimack Rivers, and into the Gulf of Maine in Massachusetts.

The summit of South Twin is open and rocky, but stands of spruce and fir trees grow in close proximity.

The Appalachian Trail, a 2,170-mile (3,500-km) National Scenic Trail from Georgia to Maine, crosses the summit of South Twin, passing from Galehead Mountain (and Galehead Hut) to the west towards Mount Guyot to the southeast.