Pedro Viloria told WPLG the Miami-Dade County officer had her two children in her auto Tuesday at a McDonald's outlet.
The incident happened at one of the restaurant's locations in Doral, Florida. The woman then fell unconscious, making her foot slip off the brake.
Viloria said the officer seemed to have trouble breathing when she came to the drive-thru window to grab her food.
"I've got to do something because she's going to crash", he said.
As the SUV rolled away with the woman and her children inside, Pedro didn't hesitate to jump out of the window and chase after the vehicle.
Viloria knew he had to make a move, so he hopped out the service window in pursuit of the SUV and the stricken driver. The woman - an off-duty Miami-Dade police officer who had her two children in the backseat - was suffering from a medical emergency.
Footage shows Viloria leaping from the window of the Doral drive-thru and as he positioned himself in front of the woman's vehicle in an attempt to keep her safe.
McDonald's released a statement applauding Viloria's actions.
"It is what a hero does, if I would've need to die to save that woman, I would've done it", the cashier said.
"First and foremost, our thoughts and prayers are with the officer and her family during this hard time", McDonald's wrote. "The real heroes are the fire rescuers", Viloria said. While the worker went for help in the restaurant, another customer who is apparently a paramedic rushed to help the officer.
What he didn't know at the time was who he was helping. The kids were okay, and the officer is still recovering in the hospital.