Police released surveillance video this week, hoping the public can help find the assailant.
Police said it started when two young girls went to the counter to order their food. A young boy cut in line and the boy's father, a man in a light-blue shirt and tan cap, emerged and argued with the girls. That's when a 16-year-old relative of the girl stepped in and began arguing with the man, who detectives say is between 35 to 40 years old.
Investigators say after the girl swore at the man, he threatened to hit her if she swore again. Cops say she cursed him again and the man punched her multiple times, knocking her unconscious. Detectives say the man's two young sons tried to calm him down. He then grabbed them and left.
The teenage victim was treated at a local hospital.
The scene is sparking outrage from those who live in the neighborhood.
"I cannot believe that. And he's a grown man," said Tameka Burnett, a McDonald's customer. "I can just see my daughter in McDonald's. That's why I don't ... I frequent this McDonald's, but we go through the drive-thru."
"I can't condone no man hitting any woman ... old or young. You know, that's just the way I was raised. We don't hit women. I mean, that's what you're told not to do," said Darryl Williams, another McDonald's customer.
Police say the suspect may have driven away in a red Dodge pickup truck with no plates.
"We're really seeking the public's help in finding the suspect and bringing him to justice," said Detective Ross Nemeroff, Los Angeles Police Department.
The teenage girl is recovering from her injuries. Her mother told Eyewitness News that she is simply traumatized and they are also asking the public to come forward with tips to help the police catch the suspect.
Anyone with information is urged to call (877) LAW-FULL or (877) 529-3855.