The 2,100-acre blaze prompted fire officials to issue mandatory evacuation orders for Sand Canyon, a community of more than 100 homes, Placerita Canyon and the Placerita Nature Center.
About 750 firefighters were on the scene and the wildfire was being fought with water-dropping helicopters.
No homes or structures have been destroyed, said Anthony Polanco, a spokesman for the county fire department.
The fire started before noon Saturday east of Highway 14. Authorities closed part of the highway and the parallel Sierra Highway.
Firefighters manned a residential area west of Highway 14 in case the fire jumped across it.
The cause of the fire was under investigation.
Meanwhile, residents were allowed to return to their homes Saturday after a stubborn fire also burning in northern Los Angeles County shifted away from rural communities.
The Pine Fire in Lake Hughes has burned 15,988 acres and by Saturday afternoon was about 54 percent contained, said Bruce Quintelier, a spokesman for the Angeles National Forest.
The blaze was one of dozens of wildfires in California during the week that burned across more than 31,000 acres.
