South of Mingus, the highway intersects and briefly travels east with Interstate 20.

A highway was originally designated on March 16, 1925 from Thurber to Lampasas, but it did not take effect until conditions were met.

[4] On March 8, 1926, SH 108 was extended north to Strawn, with a branch to Chalk Mountain added.

[5] On June 24, 1931, the section south of Strawn was redesignated as the new state-length SH 66,[6] and the rest was cancelled in exchange of mileage.

State Highway 108A was a spur routing of SH 108 that led from Lampasas east to Rosebud.