The original SH 51 was designated on August 21, 1923, as a route from
Sagerton to
Crowell, replacing a portion of
SH 28. On June 15, 1926, it was extended north to the
Oklahoma state line. On December 21, 1926, it was to extend to
Longworth, but the conditions were not met for the designation to take effect. The south end was modified several times. It was cancelled on January 19, 1932, being replaced by
SH 16. The second SH 51 was designated on April 5, 1932, as a route from Dalhart northward to the Oklahoma state line. It was extended south to
McCamey on July 16, 1934, replacing
SH 143 from
Channing to
Meadow and the majority of
SH 137 from Meadow to McCamey. On July 15, 1935, the section from Channing to
Levelland was cancelled (as it was not built yet). On December 22, 1936, this section was restored. On November 23, 1937,
SH 51 was rerouted to bypass Meadow. On February 4, 1941, the section of SH 51 from the Hockley–Terry county line to Brownfield was cancelled, creating a gap. On February 28, 1941, the section from the Hockley–Terry county line to the Hockley–Lamb county line was cancelled, increasing the gap. On May 14, 1941, the section from the Hockley–Terry county line to Brownfield was restored, partially closing the gap. On September 23, 1941, the section from Levelland to the Hockley–Terry county line was restored, closing the gap further. On April 28, 1942, it was extended south to Sheffield. On September 22, 1942, to section of SH 51 from Levelland to the Hockley–Lamb county line was restored, closing the gap. On October 24, 1956, the highway was rerouted southwest over
FM 1214 from US 290 to US 90 and
SH 227 from US 90 to the entrance of
Big Bend National Park. The section from Rankin to Sheffield was transferred to
SH 349. This entire route was canceled on August 28, 1958, when
US 385 was transferred to the west. The current SH 51 was designated on January 29, 1974; its routing has remained unchanged. ==Major intersections==