State Highway 126 is one of the original 1934 State Highways. It has also been a traveled route for many years before. Within Ventura County, the roadway was known as Telegraph Road, with the Los Angeles County section known as Henry Mayo Drive. While most of the original roadway was obliterated when the highway was widened to four lanes, portions still remain. The towns of Piru and Fillmore were bypassed early, so most of those sections remain intact. West of Santa Paula, the freeway took a new alignment, leaving the original roadway for local traffic.
Between US 99 and US 101, the roadway has slowly been upgraded and realigned. In 1963, the first section of the Santa Paula Freeway was opened from US 101 to SR-118 at Saticoy. In 1965, the remaining section east to Santa Paula opened to traffic. The freeway was planned to continue further east, but was later cancelled. Between Santa Paula and Fillmore, the roadway was widened in 1991 instead of upgraded to a freeway. The last section of two lane highway was finally widened to four lanes in 1998 from Fillmore to I-5 (US 99).