The current routing was originally planned as State Highway 17 as recently as 1922. On August 21, 1923, the SH 17 designation has been transferred to the routing of SH 17A, while this route was renumbered as SH 82. On February 27, 1958, the route was again renumbered as SH 18, to coordinate with the connecting New Mexico Highway 18 to the north. SH 18 was one of the first 25 Texas state routes proposed on June 21, 1917, overlaid on the Albany-Bronco Highway. The original proposal was a route through the Southern Plains from Bronco in Yoakum County to Lubbock, then southeast to Albany. On December 20, 1917, SH 18 had extended southeast to Cisco and replaced SH 36 to Waco. By 1919, SH 18 was rerouted through Eastland. On August 21, 1923, the route had been changed significantly. The section of the route from Spur to Albany was intact, but the previous westward route veered northward instead to Turkey, with the old route being renumbered to SH 53. The section from Albany to Cisco became part of rerouted SH 23, the section from Cisco to Eastland was already part of SH 1, so that section was cancelled, and the section from Eastland to Waco became part of the new SH 67. On August 21, 1923, SH 18 extended northeast to Memphis. On November 19, 1923, the section of SH 18 from Turkey to Memphis was cancelled, and SH 18 was rerouted east to Estelline instead. On July 13, 1925, a split from Aspermont to Anson was added. This split became part of SH 4 on December 6, 1927. On October 22, 1935, SH 18 Loop was designated through Roaring Springs. On December 22, 1936, the section from Turkey to Estelline became part of SH 86. On May 24, 1938, it was extended farther northward to the Oklahoma border, overtaking SH 88 from Turkey to Oklahoma. On January 6, 1939, two SH 18 Spur routes were designated to Spur and Peacock. On September 26, 1939, the portion from Matador to Albany was reassigned to U.S. Highway 62, SH 70, and U.S. Highway 380. The two SH 18 Spur routes were renumbered as Spur 21 and Spur 22. SH 18 Loop was redesignated as Loop 42. SH 18 was extended further north to Perryton on May 21, 1940, and on October 10, 1947 became part of SH 70 when SH 70 was extended into Matador from the south due to the rerouting of U.S. Highway 62 off of this road. SH 18A was an alternate route along SH 18 proposed on August 19, 1918. It covered a shorter route between Spur and Crosbyton during the 1920s. SH 18B was a more southerly route proposed on January 23, 1919. It was to branch off from SH 18 in Brownfield, travel southeast to Lamesa, then run due east to Albany. It is unknown if it had been built before being renumbered on August 21, 1923 as SH 83, with the section from Lamesa to Brownfield cancelled.
