The first route over the Cuyamaca Mountains was dedicated in 1912, and a plank road served as the first road across the Imperial Valley to Yuma; east of there, the Gila Trail continued east to Gila Bend. These were later replaced by U.S. Route 80 (US 80) across California and part of Arizona, and Arizona State Route 84 (SR 84) between Gila Bend and Casa Grande. The US 80 freeway through San Diego was largely complete by the time it was renumbered as I-8 in the 1964 state highway renumbering; east of San Diego, the US 80 roadway was slowly replaced by I-8 as construction progressed in the Imperial Valley. The Arizona portion of the road was built starting in the 1960s. Several controversies erupted during the construction process; questionable labor practices in Imperial County led to the federal conviction of mobster Jimmy Fratianno, and a U.S. House of Representatives subcommittee found that the Arizona government had mismanaged financial resources.

The route was completed in 1975 through California, and by 1977 through Arizona, though the bridge over the Colorado River was not completed until 1978. Since then, the freeway through San Diego has been widened due to increasing congestion, and another portion in Imperial County had to be rebuilt following damage by the remnants of Hurricane Kathleen.

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