The highway begins at U.S. Route 12 (US 12) in Silver Creek and travels east through Ike Kinswa State Park before turning south over the Cowlitz River to Mossyrock, ending at US 12.

The highway travels northeast over Silver Creek towards Lake Mayfield, becoming Harmony Road.

SR 122 continues east along the north shore of the lake and crosses the Tilton River before passing through Ike Kinswa State Park.

The highway continues east to Harmony, where it turns south to cross the Cowlitz River, eventually ending at an intersection with US 12 north of Mossyrock.

In 2011, WSDOT calculated that between 850 and 1,100 vehicles per day used the highway, mostly at the US 12 intersection in Mossyrock.