North of Swartz, LA 139 travels through rural surroundings until reaching Bastrop, where it serves as the main north–south thoroughfare on the south side of town.

LA 139 heads northeast on Old Bastrop Road as an undivided four-lane highway with a center turning lane through the Lakeshore suburb situated along Bayou DeSiard.

After a short distance, the local name changes to Lincoln Road until reaching an area known as Frizzell Spur, where the center lane is discontinued.

Five miles (8.0 km) later, LA 139 enters the Bastrop city limits at Naff Avenue and continues onto South Washington Street.

Over the next three blocks, the highway passes the Morehouse Parish jail and sheriff's office complexes before reaching the city's courthouse square.

The various routes intersecting in the center of Bastrop connect to such nearby communities as Mer Rouge, Oak Ridge, and Sterlington.

[7][8] Five years after its designation, Route 14 was chosen as the alignment of US 165 between Alexandria and the Arkansas state line when the U.S. Highway system was implemented in 1926.

With the 1955 renumbering, the state highway department initially categorized all routes into three classes: "A" (primary), "B" (secondary), and "C" (farm-to-market).