Riedl was one of the top pitchers on the Crusader baseball team from 1961-1963, leading Holy Cross to back-to-back appearances in the College World Series in 1962 and 1963. He posted a career record of 18-5 during his collegiate career, including a perfect 7-0 mark during his junior season. Riedl remains the last Holy Cross pitcher to win a game in the College World Series, when he defeated Colorado State 4-3 on June 11, 1962. He also posted victories in the NCAA District I Playoffs against Bridgeport in 1962 and Boston College in 1963, to propel the Crusaders into College World Series. In addition, Riedl threw a no-hitter against Dartmouth on April 26, 1962, and tossed a one-hitter against Amherst College on May 5, 1961.