Hancock, who pitched three innings in Saturday's game, an 8-1 Cubs win, was killed in an auto accident on westbound Highway 64/40 within the St. Louis city limits.
According to a statement issued by the Cardinals, St. Louis manager Tony La Russa informed Hancock's father of the news. Major League Baseball announced that Sunday's game between the two teams has been postponed.
Hancock, who was single, has been a member of the Cardinals since February 2006 and helped the team to its 10th World Series title last fall.
The Cardinals ask that all fans join with the team in offering their prayers and condolences to Hancock's family.
Fans on Sunday were leaving bouquets of flowers outside the gates at Busch Stadium. A note from Susan B. and Justin K. said, "To the Hancock family, Cardinals players and friends, your fans love you. We send our love."
Signs on the ticket windows said Sunday's game was postponed because of the "tragic loss in the Cardinals family."
The Cardinals have had to deal with tragedy before. On June 22, 2002, pitcher Darryl Kile was found dead in the team hotel in Chicago. The two teams had been scheduled to play a day game at Wrigley Field, and Kile had pitched just four days earlier.
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.