Gonzalez unleashed a vicious left hook straight into Abner's face, and though the 25-year-old returned to his feet he was clearly shaken and quickly sent to the canvas once again, this time for good.
The bout lasted just 155 seconds.
"This moment, this is the single greatest and most glorious moment of my life,” Gonzalez said.
"People didn’t give me the respect I deserved and treated me as a stepping-stone for Mares. They were making plans for a fight with (Leo) Santa Cruz before fighting me first.
"When I dropped him for the first time, I knew he was hurt.
"I had to go in for the kill. I didn’t want to let him survive. It was just a matter of time before I would knock him down again."
Mares was philosophical after the first defeat of his career to an opponent renowned as a big puncher but who was not expected to trouble the title holder.
"I’m OK. I have to learn how to win as well as learn how to lose,” Mares said.
"I’m in the sport not to remain undefeated, but to fight the best.
"I was all right after the knockdown, but the ref did his job and I respect him. Now I’m going to take a break and relax with my family."