He made his major league debut with the Cubs on April 20, 1948, and spent 11 years in the National League as a second-string catcher. He appeared in 608 games played over that span, and his 50 games started as a catcher with the 1950 Cubs were the most of his big-league career. The following season, on June 15, 1951, he was involved in a blockbuster trade with the Brookyn Dodgers, in which the league-leading Dodgers obtained the Cubs' slugging outfielder, Andy Pafko. But when injury sidelined Brooklyn's Hall of Fame catcher, Roy Campanella, Walker was behind the plate in the deciding game of the 1951 National League tie-breaker series on October 3, , when Bobby Thomson hit the "Shot Heard 'Round the World," costing the Dodgers the pennant.