Bailey, making his second start since being recalled Thursday from Triple-A, lasted just 3 2/3 innings and surrendered eight hits, including home runs by Albert Pujols and Rick Ankiel.
Bailey walked three and struck out two over 76 pitches.
"He's still a very young pitcher," said Reds manager Dusty Baker. "Most pitchers his age do have trouble with walks and location. He got bit by the two-out hit tonight. He's got to figure out a way to close out an inning."
Joey Votto hit his 11th home run, and Edwin Encarnacion went 3-for-4 with a double in the loss.
The Reds, coming off a 3-5 road trip, have lost six of their past nine games.
Ken Griffey Jr., who hit career home run No. 600 on Monday night at Florida, went 0-for-3 with a walk.
Griffey received multiple standing ovations from the crowd Tuesday, once when he took the field prior to the game, and again when a video replay of his historic home run was shown in the middle of the first inning, prompting Griffey to emerge from the dugout and tip his cap to the crowd.
A four-run third by the Cardinals quickly put a damper on the evening, however.
Consecutive doubles by Brendan Ryan and Ryan Ludwick put the Cardinals ahead 1-0.
Pujols then snapped an 0-for-12 skid by crushing a 1-0 pitch from Bailey, sending it 423 feet to center for a two-run homer to give the Cardinals a 3-0 lead.
"Good hitters are going to crush that ball," said Bailey. "That pitch was right down the middle."
Ankiel, who had missed the previous five games with cellulitis in his right knee, made the score 4-0 with a solo homer to right on a 3-1 pitch from Bailey.
It was the Cardinals' first set of back-to-back home runs this season, and the ninth set allowed by Reds pitchers.
In the fourth, Ludwick again doubled home Ryan to make the score 5-0.
Ludwick went 4-for-5 with two doubles, a home run and four RBIs for St. Louis.
"He found the heart of the plate too much," said Baker of Bailey. "He was throwing the ball with good velocity. He didn't have a feel for his breaking ball. They sat on his fastball. If he can't locate one or two pitches, it spells trouble."
Bailey, who made his big league debut last season by going 4-2 with a 5.76 ERA in nine starts, was outpitched Tuesday by another promising young pitcher -- Cardinals right-hander Mitchell Boggs.
Boggs, 24, allowed two runs on four hits in five innings to earn his first Major League victory in his first career start.
"He threw the ball well," said Baker of Boggs. "He had a lot of movement on the ball. I could tell the way [Cardinals catcher] Yadier [Molina] was catching the ball that it was moving. I was impressed."
Votto's 11th home run, a two-run, 410-foot blast to center, made the score 5-2 in the fourth. It was Votto's first home run since May 24 at San Diego, a span of 60 at-bats.
The Reds would get no closer.
Ludwick's solo home run, his 15th this season, on the first pitch from David Weathers put the Cardinals up 6-2 in the seventh.
His RBI single to right off Jeremy Affeldt made the score 7-2 in the ninth.
The news wasn't all positive for the Cardinals, who might've lost Pujols for an extended period of time.
The St. Louis slugger left the game in the seventh with a strained left calf muscle. After hitting a ground ball, Pujols fell to the ground just outside the batter's box holding his lower left leg, then had to be helped off the field. Pujols will be examined Wednesday.
Mike Lincoln pitched 2 1/3 perfect innings in relief of Bailey.
Adam Dunn, who went 0-for-2 with two walks and a run scored, has reached base safely in 26 consecutive games.
Bailey, meanwhile, has now issued seven walks in 10 innings in two starts and is grasping for solutions.
"You just have to find a way," Bailey said. "If one breaking ball is not working, you have to find it, or use the changeup more. I probably should've had the light bulb go off and use that pitch more, or locate my fastball better."