ˌɔn sɛptˈɛmbɚ twˈɛnti sˈɪks, nˈaɪntiːnhˈʌndɹɪd θˈɜːɾi nˈaɪn, ˌɛsˈeɪtʃ ˈeɪɾi wʌz ɛkstˈɛndᵻd saʊθwˈɛst tə ðɪ ˌæɾɐskˈoʊsə kˈɑːɹnz kˈaʊnti lˈaɪn, ɹᵻplˈeɪsɪŋ ˌɛsˈeɪtʃ θɹˈiːhˈʌndɹɪd twˈɛlv.
On September 26, 1939, SH 80 was extended southwest to the Atascosa–Karnes county line, replacing SH 312.