In three playoff rounds in 2004, Walker combined to hit .293/.379/.707 with a pair of home runs in each tournament, setting a franchise record for home runs hit by a left-handed batter in one postseason. Walker made his playoff debut with the Cardinals in Game 1 of the NLDS versus the Dodgers, homering twice and scoring four runs in an 8−3 Cardinals win. He became the first Cardinal with a multi-home run game in LDS play. In Game 1 of the National League Championship Series (NLCS) versus the Houston Astros, he was a home run short of hitting for the cycle.