The fire was first reported at 8:21 p.m. in the area of Punta del Este Drive and Colima Road.
It was first estimated at around three acres, but grew to about 15 as it spread in light-to-medium fuel.
One firefighter was injured falling down the hillside and was transported to a local hospital for treatment.
The flames were not immediately threatening any structures, but some residents in homes below the hillside were packing up and evacuating their homes as a precaution.
There are also high-tension power lines overhead in the area, though they did not appear immediately threatened by the flames.
Firefighters battling 15-acre blaze on Hacienda Heights hillside overlooking homes
Orange County firefighters assisted Los Angeles County in the firefight.
In less than 90 minutes, it appeared firefighters had the flames well under control and reduced to hotspots.
Investigators were looking into the possibility that fireworks were possibly involved in starting the fire. As AIR7 HD flew over the area, more fireworks were seen in a nearby neighborhood.