Kacer, a 29-year-old from Scranton, Pa., stuck his cap out over a railing with his right arm in Section 121A of Yankee Stadium, snatching Curtis Granderson's foul ball in the first inning.
The catch earned Kacer a large ovation from the crowd before he handed the ball over to his 13-year-old nephew, Isaiah, who was attending his first Yankees game.
"He's like, 'Get up and catch it,'" Kacer said, "'and I'm like, 'It's going nowhere near me.
"And then I watched it bounce and I'm like, 'I ... might ... be able ... to get it,' and it just happened to go right in the hat. So it makes for a great souvenir for him."
But what made Kacer's grab more impressive was the fact he is a wounded veteran, one of more than 20 on-hand for the Yankees' game against the Rockies as part of Achilles International, a New York-based nonprofit providing mainstream athletic opportunities for people with disabilities.
Kacer, who spent 11 years with the National Guard and lost his left arm in a 2008 rocket attack, was on the field with the rest of the group before the game and was greeted by outfielder Nick Swisher before taking a picture with manager Joe Girardi.
The group, wearing green T-shirts, arrived Friday afternoon from Walter Reed Army Medical Center and took a Yankee Bus from the Union League Club on 37th Street to Yankee Stadium. It was escorted by the NYPD and 60 members of the NAM Knights of America Motorcycle Club.
The wounded veterans will visit Ground Zero, the Statue of Liberty and the 9/11 Museum on Saturday before the culmination of the weekend, Sunday's 9th Annual Achilles Hope & Possibility 5 Mile Race through Central Park.
Genna Griffith's husband was wounded in Iraq in 2004, and she became inspired five years ago when singing for the USO before deciding to volunteer. Now, she's the director of the Achilles Freedom Team of Wounded Veterans, which was welcomed by the Mets last season at Citi Field before the Yankees hosted the group Friday night.
"It means the world to them," Griffith said. "All of these guys, a lot of them are sports fans. I know when I told them that they were going to the Yankees game, one guy literally picked me up, hugged me and spun me around. It shows that people appreciate their sacrifice."
Barret Swatek, the group's director of media relations, said her life and perspective have forever been changed by working with the soldiers.
"My life was never as full as it is now," she said.
Michael Fradera, a 33-year-old Brooklyn native, joined the Marines in 1998 before joining the Army four years later. On routine patrol in Baghdad as a squad leader and staff sergeant on Aug. 17, 2007, his vehicle ran over an improvised explosive device and he lost a leg.
Fradera said the military attitude of "Never Quit" gave him the strength to seek other outlets three months after his injury.
"I started seeing other veterans doing things, competing in certain events -- kayaking, cycling, scuba-diving -- and I said I want to do that," said Fradera, a lifelong Yankees fan. "And what motivates me is seeing the veterans who are in worse condition than myself who have not given up and push on every day, so that gives me the strength to keep pushing on and for me to not have any excuse to sit on my butt all day."
Matt Fortuna is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.