Contreras, 24, took Pittsburgh Pirates reliever A.J. Schugel deep to center field while pinch hitting in the sixth inning of the Cubs' 10-5 victory Sunday night. According to Elias Sports, he became the eighth Cub and 117th player overall to hit a home run in his first major league at-bat.
"I'd been dreaming of this moment (first at-bat) since I was a kid," Contreras told ESPN's Buster Olney after the game. "I knew it was gone. When I started running the bases it was exciting ... it was amazing. It was the craziest thing, the best feeling I ever had."
A fan even returned the ball to Contreras, which sat in his locker.
Contreras became the first Cubs player to homer on the first pitch he saw since Jim Bullinger in 1992.
Of course, Contreras is still a rookie, and the team's No. 3 catcher. Once the curtain call ended, he had to get back to warming up pitchers.
"That was funny," Contreras said. "After I hit a homer, they say, 'I know you're happy, but you have to go back to the bullpen.'"
Some of Contreras' new teammates tweeted their reaction to the big moment.
Contreras was hitting .350 with nine home runs at Triple-A Iowa before being called up on Friday. He entered the game that day in the ninth inning as a defensive replacement but didn't get an at-bat until Sunday. And now he'll start Monday's game catching veteran John Lackey.
"We have some things to talk about, for sure," Lackey said Sunday afternoon. "We'll try to get a system together where we can get to the pitch we want to throw quickly so the game flows good."
The key for Contreras is to keep his pitcher in rhythm. He caught Lackey in a bullpen session over the weekend and in spring training. He has prioritized two things as he came up.
"Get to know the pitcher," Contreras said. "And then get a relationship with the umpire. Get familiar with him."
Contreras is expected to be an upgrade defensively as he threw out 31 percent of runners at Iowa before being called up. By contrast, Miguel Montero has only thrown out two of 32 baserunners this year for the Cubs.
"The skill set is there," Lackey said. "It's just knowing the hitters and calling a game."
The Cubs figure Lackey could be a good debut for Contreras considering the veteran isn't complicated.
"John is pretty much predetermined, where a catcher can follow his lead," manager Joe Maddon said. "I think John is really a perfect first guy for Contreras to catch."
Lackey believes he can get the rookie to follow his lead -- and it'll involve plenty of fastballs.
"I'm going to figure out a system where I can give him a little something before I get up there," Lackey explained. "That way his talent can come through. We don't want him to be thinking too much."
Information from The Associated Press was used in this report.