From the south, LA 109 begins at an intersection with US 90 and LA 3112 in Toomey, southwest of Vinton. From this four-way intersection, US 90 heads east toward Vinton and south a short distance to I-10 (Exit 4), at which point the two highways proceed west in a concurrency across the state line into Orange, Texas. Meanwhile, LA 109 proceeds due north, immediately crossing the Union Pacific Railroad (UPRR) tracks, for and intersects LA 3063, which heads east to the Delta Downs Race Track and ultimately into Vinton. Continuing north for , LA 109 crosses the Sabine River Diversion System and curves slightly to the northwest for about the same distance to reach the community of Starks. Here LA 109 crosses the Kansas City Southern Railway (KCS) tracks and intersects LA 12 a short distance later. LA 12 heads into Texas on the west and DeQuincy on the northeast. LA 109 then proceeds due north for and crosses from Calcasieu Parish into Beauregard Parish.