The driver was airlifted to a hospital in critical condition after the late Wednesday crash on winding Glendora Mountain Road in the Angeles National Forest.
The motorist, whose name hasn't been released, was the only person in the car.
Authorities are investigating the possibility the crash was the result of a race.
City News Service says the car went over a 300-foot cliff and the driver was ejected on the way down. He was found 150 feet below the edge of the roadway.
Four teenagers were hospitalized earlier this month after their car plunged off the same roadway, which is frequently used for street racing.