Harris gave up a pair of two-run homers and took the loss as a New York Mets split squad beat St. Louis 10-7 on Sunday.
St. Louis played with a short-handed lineup. Third baseman David Freese was a late scratch after experiencing tightness in his back. Right fielder Carlos Beltran, battling a sore toe, went 1 for 5 in a minor league game.
Jon Jay was 3 for 3 with two RBIs for the Cardinals.
Jamie Hoffmann and Zach Lutz each hit a two-run homer off the 27-year-old Harris, selected by St. Louis out of the Naval Academy in the 13th round of the 2008 draft. He had been unable to play for the organization because of his military service obligation.
"It was one of those things where, if you had told me this (would happen) two or three years ago, I wouldn't have believed you," Harris said. "It's one of those learning experiences. You get humbled real quickly, but it's good. It's definitely going to jumpstart another whole level of work ethic to get where I want to be."
With the Cardinals leading 6-5 in the seventh inning, "Anchors Away" began playing over the stadium sound system when Harris took the mound to throw his warmup pitches.
"I heard it while I was out there," said Harris, who served on ships in the Middle East and off the coast of South America, among other places, and now expects to start the season in extended spring training.
"That's not a bad intro at all. It kind of puts things in perspective for you when you are out there."
Prior to the right-hander's first pitch, public address announcer John Frost introduced "Lieutenant Mitch Harris" to generous applause from the red-clad crowd.
"I got a big smile out of that," Harris said. "It means so much to me what this organization has kind of embraced. It's good to embrace that because that is something I am very proud of. I just wanted to have a little better first appearance, obviously."
New York didn't treat Harris nearly as well. Hoffmann greeted him with a two-run homer that gave the Mets a 7-6 lead. After retiring Travis d'Arnaud on a grounder, Harris walked Mike Baxter before allowing his second two-run homer, this one to Lutz.
Afterward, Cardinals manager Mike Matheny noted that he would have preferred to use Harris in a less stressful situation.
"I was real happy we got him in there," Matheny said. "He and we pictured it turning out differently—better for him. We don't know how his career is going to unfold but I can't image having many more rewarding moments in your life than having a group of more than 6,000 people honor you and for him to be on the big league field."
The Cardinals scored at least one run in each of the first four innings to mount a 6-0 lead on a windy South Florida afternoon.
Mets pitcher Aaron Laffey struggled a bit in his first start of the spring, allowing four runs and five hits in three innings. The runs were the first allowed by the left-hander this spring.
Laffey worked as both a starter and reliever for Toronto last season. After the game, he was reassigned to minor league camp.
"The first two innings I wasn't real comfortable, just kind of dropping on my back leg," Laffey said. "The last inning I thought I threw the ball really well."
NOTES: Shelby Miller and Joe Kelly will take their competition for the No. 5 spot in St. Louis' rotation into Monday's game. Miller will start against Minnesota in Fort Myers, with Kelly set to follow out of the bullpen. ... Collin Cowgill had two hits for the Mets.