Duffy was called up to the Giants and played his first game in the major leagues on August 1, 2014. However, Duffy's MLB debut is officially recognized as May 22, 2014, as the Giants' game at the Colorado Rockies that day was suspended due to rain and resumed on September 1. Duffy entered the resumed game in the bottom of the sixth inning at second base, striking out in his only at-bat. In the 2014 regular season, Duffy played in 34 games and had 60 at bats. He recorded 16 hits and 5 runs while batting .267/.302/300. In Game 2 of the 2014 National League Championship Series, Duffy scored the tying run from second base on a wild pitch in the top of the ninth inning. He recorded a speed of 20.3 miles per hour when he raced from second to home. Duffy was also one of five rookies eligible to play against the Royals in the 2014 World Series. In the 2014 World Series, at age 23, he was the youngest player on the Giants' 25-man roster. By this point in his career, the knobs of his bats were decorated by cartoon "Duffman" decals.