The five-time All-Star spent 9 1/2 seasons with the Rockies, including their inaugural season in 1995, before getting traded to St. Louis for three minor leaguers on Aug. 6.
Walker, who resides in Colorado, had previously blocked trades to Arizona and Texas over the last two years, before accepting a deal to the top team in the NL.
“They are going to the playoffs and have a chance to win the World Series,” said Walker after the trade was announced.
Walker won three batting titles with Colorado and was the NL MVP in 1997. He hit .384 with 154 home runs in 592 career games at Coors Field.
Walker is batting .298 with 10 homers and 26 RBIs in 36 games with the Cardinals, helping St. Louis clinch the NL Central and home-field advantage in the NL playoffs.
St. Louis reached 100 wins for the first time since 1985 and the seventh time in club history with Thursday’s 4-3 win over Milwaukee.
“A 100th win is a magic number,” manager Tony La Russa said. “It has significant meaning for a team. It’s a mark of excellence. You have to be really good to get that number. Ninety-nine was a great number. One hundred has a different feel.”
St. Louis is inching closer to the club record for wins in a season. The Cardinals need to go 7-3 over their final 10 games to surpass the 106 victories by the 1942 team.
The Rockies play three more games at Coors Field before ending the season with a seven-game road trip to Los Angeles and Houston.
Colorado beat Arizona 7-1 in the first game of Thursday’s doubleheader and came away with a 2-1 victory in 10 innings in the nightcap, completing a three-game sweep of the major league-worst Diamondbacks.
Rockies pitchers allowed only four runs in three games against Arizona, but Colorado manager Clint Hurdle knows St. Louis will present a much stiffer challenge to his staff.
“We played 19 innings of baseball and only gave up two runs in our park,” Hurdle said Thursday. “We will be facing a different lineup tomorrow.”