A largely rural route, LA 138 connects the small villages of Collinston and Mer Rouge, located northeast of Monroe.

The two highways split toward Monroe and the Arkansas state line, respectively, after heading west into Bastrop, the parish seat.

LA 138 proceeds northward along Doss Highway through rural farmland and crosses the Union Pacific Railroad tracks at a point known as Windsor.

[2][3][4] LA 138 runs through Collinston on Main Street, first curving due east, then making a zigzag at Keller Drive.

Continuing northeast, LA 138 passes by a local school, followed by a succession of industrial plants just outside Mer Rouge.

The route proceeds for 0.5 miles (0.80 km) before ending at an intersection with Davenport Avenue, the village's main street, which is part of US 165, US 425, and LA 2.

[6] Route 47 was designated by an act of the state legislature in 1921,[7] but it was originally projected to follow a direct path from Oak Ridge to Collinston along the former Missouri Pacific Railroad line.

[6][10] In the original Louisiana Highway system, the northern section of LA 138 from Collinston to Mer Rouge was designated as State Route C-1557.