Tyler Cravy (0-1) made an impressive major debut for Milwaukee, allowing a run on four hits in seven innings.
The Brewers won the series opener 1-0 and Milwaukee pitchers had thrown 19 consecutive scoreless innings before Reynolds' RBI.
Lynn (4-4) struck out five and walked one in 7 2-3 innings.
Kevin Siegrist got the last out in the eighth and most of a sellout crowd of 42,835 stuck around to see Trevor Rosenthal earn his 16th save in 17 chances. The Cardinals are 6-2 with one game to go on the home stand.
Matt Holliday was ejected by plate umpire Joe West for arguing a called third strike in the seventh, ending his National League-record 45-game streak reaching base safely to start the season. Derek Jeter set the major league record with 53 in a row in 1999 for the Yankees.
Manager Mike Matheny, who had gone out to aid Holliday's cause, also was ejected. That made a total of five ejections in the last five games at Busch.
The Cardinals entered averaging 2.82 runs run support in starts by Lynn (4-4), fourth lowest among National League pitchers. In 10 of the 11 starts, they totaled 22 runs.
Randal Grichuk doubled with one out in the second and scored easily on Reynolds' two-out single. The Cardinals didn't have a runner in scoring position the rest of the way, and grounded into double plays in the third, fourth and fifth.
Brewers: OF Khris Davis underwent surgery to remove torn right meniscus and will be sidelined 4-6 weeks.
Cardinals: P Adam Wainwright, lost for the season with a torn left Achilles in April, has been at Busch during the home stand.
Jimmy Nelson is 0-3 with a 10.24 ERA against St. Louis in three games. John Lackey is 5-1 with a 1.83 ERA at Busch in 10 starts and struck out nine in seven scoreless innings his last time out against the Dodgers.
Three Brewers chased futilely for Matt Carpenter's foul pop in the fourth and two of them - shortstop Jean Segura and third baseman Aramis Ramirez - tumbled into the stands. No one was injured.
Link to box score: St. Louis (34-18) 1, Milwaukee (18-35) 0