McCullers threw 76 pitches in five innings, giving up four hits and allowing just one run on a Salvador Pérez solo shot in the fourth.
Aside from that pitch, and a couple of others he might want to have back, McCullers was sharp. His changeup would buckle Royals hitters, the command on his fastball was strong and his breaking ball was improved, manager Dusty Baker said.
The Astros defeated the Royals 6-3 in the first of two exhibition games in as many days.
Houston's run support came from an RBI-double by Alex Bregman in the first inning; a pair of two-run blasts by outfielder Josh Reddick and catcher Martin Maldonado in the fourth inning and Myles Straw scored on a wild pitch in the seventh for good measure.
For McCullers, this wasn't just an exhibition game. He's been waiting on this moment for nearly two years, since undergoing Tommy John surgery in November 2018.
McCullers forever earned the respect of his teammates that year after pitching through the regular season and in relief in the playoffs despite his right-elbow discomfort.
The 26-year-old Astros starter has been building up to start the season since spring training was shut down in March due to the coronavirus pandemic.
Then he arrived to summer camp at Minute Maid Park earlier this month, only for testing hiccups to force the cancellation of critical workouts in a shortened 60-game season.
Finally, on Monday, McCullers was able to take the mound at a Major League ballpark and face hitters in jerseys with different logos. And he didn't disappoint.
McCullers retired the first seven Royals hitters with ease. Then in the fourth inning, he struck out right fielder Hunter Dozier and designated hitter Jorge Solar before Perez stepped to the plate.
McCullers said he tried to get cute with his changeup in the Perez at-bat, after logging a couple of quick strikeouts.
"I was happy with my ability to mix pitches and (Maldonado) doing a really good job back there staying on me and making sure I was using all of my weapons," McCullers said.
"I got stubborn and wanted to throw one for a strike again. I don't know if he was looking for it. I don't know if it's because I was throwing heaters (inside), he was looking for that heater in and kind of caught the changeup out in front. But other than that ... I was happy with the ability to throw it for strikes, the ability to expand the zone with it."
For McCullers, who is not a fan of flying, getting some game action was important. He'd been working on details of his pitches with Astros pitching coach Brent Strom and bullpen coach Josh Miller, and last week was the first full, uninterrupted week of preparation.
McCullers wanted to see how it all culminated.
McCullers woke up Monday morning a little anxious and nervous.
Flying to another city on the morning of a start was also different, McCullers said, but he felt good about the game plan he'd put together with Strom and Miller.
Once he arrived at Kauffman Stadium, where the Astros went immediately after landing in Kansas City, McCullers got into his gameday routine and settled down.
There had been a high chance of rain in Kansas City, which jeopardized the game, but the Astros and Royals were able play ball without a single rain drop.
"The word of the day for me is just grateful," McCullers said. "I was very grateful for the stars to align and for us to play tonight with the weather that was possible coming through. I'm grateful for my teammates and the opportunity to pitch on a big league mound."
The Astros are back at Kauffman on Tuesday afternoon with Josh James taking the mound.
McCullers pitches again Saturday in the Astros' second regular season game against the Seattle Mariners at Minute Maid Park.
Justin Verlander will take the mound Friday in the Astros' opener.