The route was designated on August 27, 1959, as a renumbering of SH 235 to match the numbering of Louisiana Highway 12 (LA 12), with which it connects.

SH 12 begins at an intersection with I-10 and US 90 on the north side of Vidor, and travels to the northeast.

It travels through lightly populated country before reaching the Sabine River at Deweyville.

'[2] In 1917 the route followed present day Interstate 10 eastward from El Paso to roughly Esperanza.

It was proposed from there to hug the Rio Grande through Big Bend and towards Langtry (again, never built) where it would have met up with present day U.S. Highway 90.

[3] Many of the roads in West and South Texas did not exist as planned (and still do not), and the technology available in 1919 was not up to the task of building so many miles of non-existent roadway quickly and efficiently.

[4] In late July 1917, the segment from El Paso, to near Lobo was rerouted to instead go to a proposed New Mexico crossing, due south into Van Horn, via SH 54 and continued to near Valentine, Texas over US 90.

Much like other highways at the time, multiple alternate routings have been created using the same number, and SH 12 was no exception.

On March 15, 1920, The Rio Grande hugging Big Bend segment was completely abandoned, with SH 12 now using the current US 90 highway from Valentine to Alpine to Sanderson.

On August 21, 1923, SH 12 had lost much of its assignment, being mostly confined to south and southeast Texas.