The blaze that erupted at about 3 p.m. near the town of Valyermo in northeast Los Angeles County was 100 percent contained late Friday, U.S. Forest Service spokesman Robert Brady said.
The fire prompted an unknown number of residents to evacuate their homes voluntarily, Brady said.
KCAL-TV showed horses fleeing stables as flames and a thick layer of smoke covered a wide grassy hillside in the early stages of the fire.
One Forest Service firefighter was injured and was taken by helicopter to Antelope Valley Hospital, Brady said.
More than 150 firefighters from both the Forest Service and the county fire department and three water-dropping helicopters battled the blaze that was burning about 50 miles northeast of Los Angeles and 20 miles southeast of Palmdale.
Crews will remain on the scene throughout the night to mop up hot spots, Brady said.
Earlier Friday, firefighters surrounded a 28-acre blaze in Riverside County south of Lake Elsinore that investigators believe was started by two boys playing with fire.
The blaze broke out Thursday evening and briefly threatened a half-dozen homes in Wildomar, but flames moved away from structures during the night, said Riverside County fire spokeswoman Cheri Patterson. It was fully contained by 11:30 a.m. Friday.
Investigators interviewed the two boys ages 12 and 13, decided they did not have malicious intent, and did not plan to file any arson charges, Patterson told the Riverside Press-Enterprise. But the boys' parents could be responsible for helping to cover firefighting costs.
Another 13-year-old boy, Anthony Ramos, said he and his friend were riding scooters when they spotted the fire on the hill and asked a neighbor to call 911.
"We got panicked," he told the Press-Enterprise. "Thank God the wind was blowing the other way."
Ramos said he saw the two boys believed to have started the fire running down the hill and crying.
"They were scared," he said.