Although NH 125 is mostly an undivided two-lane highway, it regularly carries heavy truck and tourist traffic, especially in the summer months, when it is used as a toll-free alternative to Interstate 95 and the Spaulding Turnpike.

NH 125 stays to the west of town and is initially a four-lane divided highway serving as the local shopping strip.

The highway turns northeast to bypass downtown and crosses NH 27 and the Lamprey River, narrowing back to two lanes thereafter.

As NH 125 continues north, it begins to parallel the Salmon Falls River (which forms the border between New Hampshire and Maine) to the west, and the Spaulding Turnpike to the east.

The highway meets the eastern terminus of NH 75, which connects to the nearby turnpike at exit 17, before passing through downtown Milton, continuing to run along the western side of the Salmon Falls River.

After leaving the downtown area, NH 125 turns slightly westward away from the river and continues winding north, running nearly adjacent to the turnpike.

As it approaches the Branch River, a tributary of the Salmon Falls, NH 125 turns west and interchanges with the Spaulding Turnpike again at exit 18.

This interchange marks the northern terminus of the Spaulding Turnpike, and the White Mountain Highway designation continues northward on NH 16.