NH 128 runs from the Massachusetts border in Pelham northward to Londonderry, south of Manchester.

NH 128 is named Mammoth Road throughout its entire length.

The southern terminus of NH 128 is at the Massachusetts state line in the town of Pelham, where the road continues into Massachusetts as an unnumbered road in the town of Dracut.

The northern terminus is at New Hampshire Route 28 in Londonderry.

Mammoth Road continues northward along NH 28 and then New Hampshire Route 28A.