On July 24, 2009, the A's traded Holliday to the St. Louis Cardinals for prospects Brett Wallace, Clayton Mortensen, Shane Peterson and $1.5 million. He represented an instant offensive upgrade for Cardinals left fielders, who had batted .212 with a .294 on-base percentage and a .342 slugging percentage in 2009, and for cleanup hitters, who had batted .250 with .325 on-base percentage and .443 slugging percentage. He hit in the fourth slot in the lineup behind All-Star first baseman Albert Pujols. Because Pujols already wore jersey number 5, the number Holliday previously wore with Colorado and Oakland, St. Louis issued him the number 15.