With North Carolina facing a fourth-down and four from the 4-yard-line in the final two minutes, Johnson raced onto the field from the sidelines just before North Carolina quarterback Bryn Renner took the snap.
Johnson got onto the field just in time to try and jam Erik Highsmith from taking the inside route to the goalpost. But Highsmith got the jump on him.
Renner delivered the ball and for a split second it appeared Highsmith was going to make the touchdown catch and give the Tar Heels a lead. But at the last second, the 5-foot-9, 187-pound Johnson stuck his hand in and knocked the ball loose.
Louisville took over and ran the clock out for a 39-34 victory.
"They beat us earlier on that same play," Johnson said. "Same route, same receiver, so I had a feeling they were going to come back to it. The whole time I had my hand on him and watching film, we knew they were great jumpers.
"I used all of my might to try and strip the ball. I ripped down at the ball to knock it down and then I looked for a flag. I was hoping they wouldn't throw a flag."
There was no flag and Johnson made the game-saving play.
He said going goal line in practice every day and having to go against DeVante Parker and Andrell Smith prepared him for the situation.
"I'm always going against the bigger guys," Johnson said. "I just knew I had to get my hand in there somehow and knock down that ball. I had to do it.
Several of Johnson's teammates weren't surprised he came up with the big stop. He finished the game with four tackles, including a tackle for a loss.
"He does it all the time in practice," defensive end Marcus Smith said. "He makes big plays all the time, so I knew he could do that in a game."
U of L coach Charlie Strong noted the Cardinals were using a coverage they call "thief" during the last play and Johnson had Highsmith in single coverage.
"Everyone else was doubled," Strong said. "They key was doubling the tight end because he was catching a lot of balls. We wanted to make sure he was doubled and we wanted to make sure the back coming out of the backfield was covered, so we had a backer on him. They ran a crossing route, which is what we thought they might do, and Drew was able to pull it out."