The fire broke out around noon when a building caught fire near the intersection of Magnolia and Commonwealth avenues, sending a plume of smoke into the air. Firefighters battled the blaze just yards from the airport.
Firefighters on the ground and atop an aerial ladder used hoses to douse the flames.
"If we were to have any kind of wall collapse, it would collapse onto airport hangers," Fullerton Fire Deputy Chief Michael Meacham told reporters.
Employees at Capri Shoes said they noticed smoke in the back of the building. The two dozen employees inside evacuated safely before flames erupted.
"All of our employees are accounted for and that's, you know, the most important thing for us," said Stephanie Navarro, owner of Capri Shoes.
Ana Torres, who has worked for the specialty shoe company for 24 years, became emotional while recounting the incident.
"I see the smoke and I ran. I don't know what happened," Torres said when asked what she saw.
As five agencies responded to the four-alarm fire, operations at Fullerton Airport were stopped.
Departing flights were canceled and the airport was closed to arriving flights due to the fire.
Airport manager Brendan O'Reilly told Eyewitness News that they closed the runway as a precaution because smoke was billowing across the final approach and hoped to reopen it shortly.
Just before 1:30 p.m., the FAA said the airport was operating as normal.
Witnesses across Orange County reported seeing black smoke pouring from the building.
"I walked to the corner, and it grew. The flames must have been like a couple of stories high," witness Terry Hughes said.
"I mean it's right behind the airport, so I thought you know, who knows, jet fuel or whatever else might be in there," witness Raymond Castillo said.
Heavy fire through the roof initially forced firefighters to attack the blaze from outside the building.
"We requested multiple alarms and got agencies from Anaheim, OCFA, Brea, Orange and our Fullerton units," Meacham said.
Firefighters, wearing roughly 100 pounds of protective gear, ultimately contained the fire.
There were no immediate reports of injuries.
Investigators are working to determine the cause of the fire.