The road, located in southeastern Union County, begins at NM 402 between Amistad and Clayton and passes eastward through the town of Sedan to the Texas state line where the roadway continues as Texas State Highway 102 (SH 102) to U.S. Route 87 (US 87) near Dalhart, Texas.

The 8.1-mile (13.0 km) two-lane road begins at a t-intersection with NM 402 south of Clayton and west of Sedan in rural Union County.

After leaving Sedan, the highway traverses more farmland before terminating at the New Mexico–Texas state line just west of Farm to Market Road 296 (FM 296).

[4][unreliable source] In the following decade, the road was extended southward along NM 18 then westward through Bueyeros and eastward to the state line meeting up with Texas's FM 808 which was then re-signed continuously as SH 102 in 1955.

Geographic data related to New Mexico State Road 421 at OpenStreetMap