The curtain call concept was new to him.
"A couple guys in the dugout said, 'They want you to go back out there,'" Heyward said. "So I did. It was a good feeling."
Heyward's homer backed a strong six-inning pitching performance by Carlos Martinez as the Cardinals won their fourth in a row.
Matt Carpenter drove in a pair of runs for the NL Central champions.
Zack Cozart homered for the Reds, who have lost three in a row and six of seven.
Cincinnati speedster Billy Hamilton left in the eighth inning after beating out an infield single. He is day-to-day with irritation in his right groin, manager Bryan Price said.
Heyward was acquired from Atlanta in November to replace outfielder Oscar Taveras, who was killed in an automobile accident.
Heyward homered in his 42nd at-bat of the season, connecting in the third inning.
"He's a big boy with a big swing," said Carpenter, who had two hits. "Hopefully he's got a lot more homers left in there for us."
St. Louis manager Mike Matheny has final approval on all curtain calls.
"They usually look down towards my end of the dugout to see if it is appropriate or not," Matheny said.
Matheny gave the thumbs-up and the celebration was on.
Martinez (1-0) allowed one run and three hits, striking out four and walking two.
Trevor Rosenthal picked up his fourth save in as many opportunities. He struck out Joey Votto with two on to end the game.
Homer Bailey (0-1) surrendered five runs and nine hits over 5 2-3 innings. He made his first start after beginning the season on the disabled list while recovering from surgery on a torn flexor tendon.
"Early on, I felt I made some pretty good pitches that got hit," Bailey said. "They put some pretty good swings on them. But it does feel good to get back out there."
Martinez picked up his first win of the season and need just 86 pitches. He retired 10 of the last 11 batters he faced.
"I tried to limit my pitches so I could last longer," Martinez said. "It worked."
Carpenter, who has six successive multihit games, improved to 17 for 29 (.586) lifetime against Bailey. His two-run double in the sixth pushed the lead to 5-1 and chased Bailey.
Matt Adams put St. Louis in front with an RBI single in the first. He drove in Matt Holliday, who doubled with two out.
Holliday left the game after scoring with stiffness in his back. Holliday, who has hit safely in all 10 games this season, is day-to-day.
Yadier Molina made it 3-0 with a run-scoring double in the fourth. Molina has eight hits in his last 15 at-bats.
Cozart, who had three hits, pushed his hitting streak to seven games.
"We kept coming at them, we hit some balls hard," Price said. "But we weren't able to string together the big inning that we needed."
Jhonny Peralta of the Cardinals had a nine-game hitting streak snapped.
Reds: Bailey came off the disabled list for the start. RHP Pedro Villarreal was optioned to Triple-A Louisville.
Cardinals: Announced after the game that OF Randal Grichuck will be put on the 15-day disabled list Sunday with a back strain. They will recall Dean Anna from Triple-A Memphis, who is expected to be ready for the game. ... LHP Jaime Garcia threw to hitters on Friday at extended spring training. General manager John Mozeliak indicated that Garcia is still about six weeks away from returning to the roster.
Reds: RHP Mike Leake (0-0, 4.97) will make his 150th career appearance in the final game of the three-game series Sunday night. He has four career home runs and led NL pitchers in batting average in 2010 (.356) and 2012 (.305).
Cardinals: RHP Adam Wainwright (1-1, 2.08) will make his third start of the season. He has not allowed an earned run in his last 17 innings against the Reds.