Playing college baseball for the Ole Miss Rebels, Lynn set the school's single-season (146) and career (332) pitching strikeout records. The Cardinals selected him in the first round of the 2008 MLB Draft from the University of Mississippi. In 2009, he was named the Cardinals organizational Pitcher of the Year after playing in three levels and totaling a 2.85 earned run average and 124 strikeouts in innings pitched. He made his MLB debut on June 2, 2011, was a member of the Cardinals' World Series championship team that year, and a National League All-Star in 2012.