Firefighters were hoping to rein in the blaze before temperatures spike later in the day.
An enormous plume of smoke was visible across much of southern California after the fire broke out on Wednesday afternoon in dense forest land.
The blaze exploded in size within hours, including some areas that have not burned since 1968, fire officials said.
By nightfall, the flames had consumed more than 15.5 square miles of timber and shrubs. There was no containment.
About 100 rural homes were evacuated in the Lake Hughes area of the Angeles National Forest, about 60 miles north of LA.
Los Angeles County Deputy Fire Chief David Richardson said late on Wednesday that some outbuildings may have been destroyed.
Early morning TV news footage showed several structures reduced to ash.
The area is expected to have temperatures of about 35C on Thursday and higher through the weekend.
