The officer's name hasn't been released, but he's been on the force for 38 years.
He was just about to head back to his patrol car Sunday evening when he saw a man jump out of a car and start shooting at the security guard.
The officer pulled his service weapon, a Glock, from its holster and shot the first gunman, who fell to the ground.
A second gunman got out. The officer took a split-second aim, fired at him, and the suspect fell to the ground.
The President of the Garland Police Officers Association told FOX 4 that if the officer hadn't done what he did, hundreds of people would have died.
The officer had recently transitioned from motorcycle patrol to a squad car.
He declined an interview with FOX 4 on Monday.
The Association's Facebook page was full of kudos from citizens.
One post read, "The officer dropped both shooters, bravo, give that man a medal." Others posted, "Good job guys" and "Way to go boys."
Douglas Athas, the Mayor of Garland, couldn't be more proud.
"I don't find it really that surprising and I happen to know the gentleman that this was, and I still don't find it surprising," said Athas.