Miller sparked St. Louis' six-run first inning against Gray with a two-run double that glanced off the glove of diving first baseman Joey Votto and bounced down the line. He hit a two-run shot in the second, an RBI single in the fifth and another two-run homer in the eighth, clanging a drive off the foul pole in right.
Despite his big day, Miller was more impressed with the team-wide approach at the plate.
''We jumped on a really good pitcher and had nine good at-bats in a row,'' he said. ''We set the tone early. That was awesome. I think the biggest thing it takes is a bunch of quality at-bats in a row. I thought that was the best string of at-bats maybe we had all year, especially with Sonny. Everybody was able to pick out a good pitch and lay off the stuff he wanted us to chase.''
Kolten Wong went 4 for 4 and scored four times as the Cardinals set season highs for runs and hits with 23. St. Louis has won three straight games after a four-game losing streak.
It was the worst loss of the season for the free-falling Reds, who have dropped three straight and four of five. They have been outscored 33-8 during the losing streak.
St. Louis left-hander Kwang Hyun Kim (2-0) pitched five innings of three-hit ball, extending his scoreless streak against the Reds to 11 innings over two starts. No Reds baserunner got past second base against him.
Kim's main concern was staying warm and focused during the lengthy first.
''In a game like today, the first inning was very important,'' he said. ''In today's game, you really need to focus. You just don't know what can happen. During our offense, I played catch twice in the batting cage and after I pitched the first inning, what I thought was, since our offense got me a lot of runs, I shouldn't walk anybody.''
Votto greeted Ryan Helsley with a leadoff home run in the sixth. Curt Casali added an RBI double in the ninth.
Archie Bradley and Brian Goodwin each made their debut for Cincinnati after they were acquired in a pair of trades on Monday. Bradley pitched 1 2/3 scoreless innings, and Goodwin went 1 for 3.
Gray (5-2) had been one of baseball's best pitchers so far this season, but he recorded just two outs in his worst start of the year. He permitted six runs and five hits, struck out one and walked three.
''There's a reason all that stuff happens, and it's solely on your shoulders and that is very uncomfortable and it's a (bad) feeling,'' Gray said.
Miller's hit drove in Wong and Tommy Edman. Dexter Fowler and Wong each hit a two-run single before Gray was replaced by Lucas Sims.
Gray allowed six runs in an inning for just the second time in his career and first since 2016 for Oakland.
Eleven Cardinals batted in the 27-minute first inning.
Since sitting out three straight games while healthy for the first time in his career, Votto is 8 for 15 during a five-game hitting streak with a double and three solo homers.
Cincinnati infielder Matt Davidson made his second pitching appearance in three days. He also finished the Reds' 10-1 loss to the Cubs on Sunday.
Cardinals: Wong was hit in the right foot by a Nate Jones pitch in the sixth. He hobbled to first base and stayed in the game to run the bases before being removed.
Cardinals: RHP Johan Oviedo (0-1) makes his first career appearance against the Reds on Wednesday night.
Reds: RHP Tyler Mahle (1-1) is 1-3 with a 5.40 ERA in six career starts against the Cardinals. He is 0-3 over his last five starts against St. Louis.
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