The fire broke out on Saturday afternoon and spread across nearly 162 hectares of hilly terrain in northeast Los Angeles, as southern California recorded near-record temperatures.
About 400 firefighters are battling the blaze using water-dropping aircraft.
Dozens of guests at a wedding party at a mountain campground were air-lifted by helicopter after the fire cut off their exit trail. No injuries were reported.
City officials for Sierra Madre, about 32km from Los Angeles, said between 400 and 500 homes were under a mandatory evacuation order.
"A lot of that area hasn't burned in 40 years, but the weather has been on our side," Elisa Weaver, a city spokeswoman, adding that only a small storage shed had been burned.
Officials are still investigating the cause of the fire.
Late last year, strong winds, high temperatures and parched brush after a record drought caused a series of blazes from Santa Barbara to the Mexican border, destroying thousands of buildings and driving hundreds of thousands from their homes.