It serves the communities of Vernal, Lapoint, Neola, Cedarview, and Roosevelt.

Starting in Roosevelt at the intersection with US-40/US-191 (200 East), the route travels west for approximately one half mile, before turning northwest, then turning north through Cedarview to Neola.

In Neola, the route makes a right turn onto 9000 North and begins travelling east.

[4] A short piece at the east end was redesignated State Route 245, from Vernal west to the bridge over the Highline Canal west of Maeser, but the more rugged section between the bridges was removed from the state highway system.

[5] This split was undone in 1969, when SR-121 again became a continuous route from Roosevelt to Vernal.