The fire was reported around 9:30 p.m. at Security Public Storage, 13650 Imperial Highway.
Multiple agencies assisted Santa Fe Springs in battling the fire and it took hours for firefighters to get all the hot spots under control.
"It was a very stubborn fire," said Santa Fe Springs Battalion Chief Mike Glaze. "It looks like it started on the first floor and extended up to the second floor, the storage units up there, and began to run the attic."
The financial value of the damage to the building and contents is still being evaluated. No injuries were reported.