It runs 32.44 miles (52.21 km) in a north–south direction from LA 14 west of Lake Arthur to U.S. Highway 190 (US 190) east of Kinder.

The highway bisects the sparsely populated Jefferson Davis Parish as it zigzags along rural section line roads.

Roughly halfway along its route, LA 99 passes through the town of Welsh and an interchange with Interstate 10 (I-10), the region's main east–west highway.

Signs direct motorists to the nearby Lacassine National Wildlife Refuge located to the south.

LA 99 proceeds due north as an undivided two-lane highway in a rural area of southern Jefferson Davis Parish.

After another 2.0 miles (3.2 km), the highway curves east briefly then back to the north amidst a small cluster of farm houses.

US 90 passes through town on Russell Avenue, its primary east–west thoroughfare, connecting with Jennings to the east and Lake Charles to the west.

Now following the town's primary north–south thoroughfare, LA 99 passes a series of historic brick storefronts and crosses the BNSF/Union Pacific Railroad line at grade.

LA 99 immediately passes through a spread diamond interchange with I-10 at exit 54, crossing over the interstate in the process.

The two highways proceed north concurrently for 1.0 mile (1.6 km), at which point the road continues straight ahead as LA 3086.

The section from the southern terminus west of Lake Arthur to the present intersection with LA 102 southeast of Fenton made up the majority of State Route 105.

This eliminated several right-angle turns along the route but also necessitated a brief jog onto US 90 in order to proceed north onto Adams Street.