The shooting happened around noon on Topanga Canyon Boulevard, according to the Los Angeles Police Department. The man suspected in the shooting had walked into the restaurant and asked for water, which was given to him, but then some sort of argument ensued.
Employees who saw it all happen said it seemed like there was "something off" with the shooter.
"He flicked us off, acting like he was going to pull out a gun... but actually pulled out a real gun and just started shooting at all the windows and we just all ducked really fast ," said Eli Levi, who works at Shake Shack.
Fourteen employees and five customers were inside at the time, including a 10-year-old.
The suspect fired about eight times from outside, shattering several windows, but nobody was hurt, police said.
"We're lucky. We're very lucky that no one was hit or hurt," said LAPD Detective Acevedo.
The suspect was arrested a short time later walking along a nearby alley. It's unclear if he has a criminal history or if he lives in the area.