Duncan died Friday, seven years after his initial diagnosis of glioblastoma, the same cancer that killed his mother, Jeanine Duncan.
"We are deeply saddened by the passing of Chris Duncan after his courageous battle against brain cancer," the Cardinals announced on Twitter Friday. "We extend our heartfelt sympathies to his wife, Amy, the entire Duncan family, and his many friends."
Chris Duncan's father, Dave Duncan, is a former catcher and pitching coach for the Cardinals, and his brother, Shelley, is a former player for the New York Yankees, Cleveland Indians and Tampa Bay Rays.
The St. Louis Post-Dispatch said Chris Duncan was instrumental in the Cardinals' 2006 World Series win. He hit 22 home runs that season and the injury-plagued team named him rookie of the year.
"We wouldn't be here without him," manager Tony La Russa said during the series.
Chris Duncan's Major League Baseball career ended in 2010 after he was traded to the Boston Red Sox, but he only played in the minor leagues.
During his five-year career, he had a .257 batting average, 55 home runs and 175 RBIs.
In 2011, Chris Duncan became a radio host for WXOS, an ESPN affiliate in St. Louis.
He is survived by his wife, father and brother.