It continues the route of Texas State Highway 12 (SH 12) from the Newton County community of Deweyville into Calcasieu Parish, Louisiana.

LA 12 heads east as an undivided two-lane highway through a largely uninhabited and heavily wooded area in the northwestern portion of the parish.

East of Starks, LA 12 curves northeast and runs parallel to the Kansas City Southern Railway (KCS) tracks for the next 13 miles (21 km).

After passing the local middle and high schools, the route curves due east into town on West 4th Street.

Signs direct visitors to the DeQuincy Railroad Museum, located one block to the north at the convergence of two branches of the KCS Railway.

The eastern terminus is located at an interchange with US 171 and US 190, specifically at the foot of ramps leading to and from southbound US 171, heading toward Lake Charles.

Eastbound US 190 continues straight ahead toward Kinder, first connecting to ramps leading to northbound US 171 and westbound US 190, which travel concurrently to DeRidder.

[10] This cross-state route continued eastward from Ragley along the modern corridor of US 190 through Opelousas, Baton Rouge, and Hammond to Covington.

[2] Perhaps the most significant improvement to the pre-1955 route occurred in 1938 upon completion of the Deweyville Swing Bridge over the Sabine River, replacing an existing ferry service at the Texas–Louisiana state line.