Beltran also hit an RBI single in the first against Ryan Vogelsong in a matchup of the teams that won the past three World Series — and a rematch of last fall's NL Championship Series won in seven games by the Giants.
Hunter Pence hit a solo homer in the second and Pablo Sandoval homered and hit an RBI single. He also played more strong defense for San Francisco on a day Vogelsong (0-1) struggled to consistently command the strike zone.
Posey received his NL MVP plaque in another highlight-filled pregame ceremony, and the Giants will have yet another day of celebrating before Sunday's game with the World Series ring presentation.
Posey also was honored for his batting title and Silver Slugger award. Kevin Mitchell, the 1989 NL MVP, and 2000 MVP Jeff Kent took part, along with former MVP winners and Hall of Famers Willie Mays and Willie McCovey.
St. Louis' pitchers faced baserunners every inning but limited the damage.
Miller (1-0) walked Pence with one out in the sixth, then Randy Choate relieved and walked Brandon Belt on four pitches before pinch-hitter Joaquin Arias lined the first pitch he saw into a double play to first baseman Allen Craig.
Miller, the Cardinals' top pitching prospect, allowed four hits and two runs in 5 1/3 innings. He struck out four and walked four in the first road start of his career.
He went 2-0 with a 3.94 ERA in three spring training starts and five overall appearances, and faced San Francisco twice in relief during last fall's NLCS.
The Cardinals earned their second win of the year, giving their pitchers plenty of offense a day after getting three-hit by Barry Zito and two relievers in a 1-0 loss.
After Jon Jay's foul popup to start the game, Carpenter reached on an infield single and advanced on a passed ball during Holliday's strikeout. Carpenter reached third on a wild pitch as Craig batted. Craig, who had two of his team's three hits Friday, walked and Beltran singled home Carpenter.
The earned run ended a stretch of 26 straight innings without allowing one to begin the year for San Francisco's starters. Vogelsong needed 28 pitches to get through the inning.
After that, Vogelsong allowed Kozma's one-out single in the second before retiring the next eight St. Louis batters in order.
World Series MVP Sandoval homered leading off the eighth for his second of the year. Posey followed with a double for his lone hit of the game on a day the first 40,000 fans received his bobblehead doll - just about everybody in the sellout crowd of 41,402.
Notes: Cardinals 3B David Freese, on the disabled list with a strained lower back, went 1 for 4 with a double and two RBIs on Friday in a minor league rehab game with Triple-A Memphis against Oklahoma City. The hope has been that he would return in time for the home opener Monday against Cincinnati. ''We'll see how things go,'' manager Mike Matheny said. ''He's going to do some extra running today and really put a good pounding on his back.'' ... Belt lost 11 pounds to get down to 218 with his stomach bug earlier in the week. He last threw up Wednesday, but the 1B was still working to get back his strength. He has been eating protein bars between innings. ''That's the battle, getting the weight back,'' Belt said. ... San Francisco's win Friday came on Angel Pagan's bases-loaded walk, the first time the Giants won a 1-0 game via an RBI free pass since Dick Groat drew one in the top of the 21st inning on Sept. 1, 1967, at Cincinnati, the Giants said, according to Elias. ... Giants' Double-A Richmond pitchers Jack Snodgrass and Daryl Maday combined on the first no-hitter in team history Friday night in the second game of a doubleheader vs. New Britain.
Link to box score: St. Louis 6 at San Francisco 3