State Route 120 begins south of the city of Fallon, at the intersection of US 95 and Pasture Road.

From there, the route follows Pasture Road eastward through agricultural and farmlands south of Fallon.

After about 3 miles (4.8 km), the route comes close to the northern shore of Carson Lake.

At this point, the highway turns northward to traverse through more agricultural areas.

SR 120 reaches its terminus at the southwest corner of Naval Air Station Fallon, at the intersection of Pasture Road and Berney Road (SR 119), although Pasture Road continues north to access the airbase.