Driving from Bastrop to College Station, Texas
SH 21 was one of the original 25 routes proposed in Texas on June 21, 1917 along a route from the Louisiana border east of St. Augustine to Gonzales, overlaid on top of the Gonzales-St. Augustine Highway. There was proposed extension southward to Karnes City on February 18, 1918.[3] On July 16, 1923, the terminus was shortened to Giddings, with the section south of there being cancelled.[4] A spur, SH 21 Spur, was designated on March 19, 1930 from Milam to Hemphill. On August 1, 1930, this spur became part of SH 87. On September 29, 1933, SH 21 extended to Lockhart. On July 15, 1935, the section from Giddings to Lockhart was cancelled, and SH 21 was rerouted though Lincoln, replacing part of SH OSR, and followed SH 44 to Giddings. On October 21, 1936, SH 21 Spur to Chireno was added. On May 18, 1937, the spur in Chireno became a loop, SH 21 Loop. On February 21, 1938, another SH 21 Spur to McMahan's Chapel was added. On April 19, 1938, the section of SH 21 from Lincoln to Giddings was cancelled, and SH 21 extended to Bastrop, replacing part of SH OSR. On September 26, 1939, the section from Paige to Bastrop was cancelled, as it was already part of US 290. The spur and loop became Loop 34 (Chireno) and Spur 35 (McMahan's Chapel). On August 2, 1943, the western terminus had been extended to end in San Marcos, along its current route, replacing part of SH OSR.