The western terminus of NH 124 is in Marlborough at New Hampshire Route 101.

The road continues through Marlborough, a short section of Troy and enters the northwestern part of Jaffrey.

The road skirts the southern slopes of Mount Monadnock, across Jaffrey, and into Sharon.

In Sharon, there is a New Hampshire historical marker (number 68) on the northern side of the road marking the site of a gate that once collected tolls for the 3rd New Hampshire Turnpike, which followed much of the present-day route of NH 124.

The eastern terminus of NH 124 is at the Massachusetts state line in Mason.