It spans 51.76 miles (83.30 km) and is signed north and south.

on 21st Avenue in Covington, LA 21 is multilaned, in places divided by a median or by a center turn lane, with some control of access.

[1] Through most of Covington, LA 21 is in a mostly two-lane undivided highway concurrent with US 190 Bus., which returns to US 190 on the east side of Covington—an area known as Claiborne Hill.

Between Bush and Sun, LA 21 widens to a four lane, divided highway after merging Louisiana Highway 41, continuing due north from the merge.

LA 21 then runs north through Varnado and Angie before becoming Mississippi Highway 35 at the state line.