On June 7, 2014, Giles was promoted to the major leagues after the Phillies placed Mike Adams on the disabled list (DL). Four days after the promotion, he made his major league debut in a game against the San Diego Padres, and relinquished a home run to his first batter, but then worked through the inning to preserve a lead for the Phillies, who won the game. His first winning decision came on August 10 against the New York Mets. On September 1, Giles was part of a combined no-hitter against the Atlanta Braves; Cole Hamels, Giles, Jake Diekman, and Papelbon did not allow a hit in the game's nine innings. Later that month, Giles picked up his first save as a major league pitcher in a game against the Oakland Athletics on September 20. Giles ranked among the best relievers in the major leagues in many key statistical categories relating to strikeouts, ERA, and walks plus hits per innings pitched (WHIP). He also represented a bright spot for the Phillies – and a point of pride for the fan base – during a rebuilding phase for the team. Adam Dembowitz of Crashburn Alley, an ESPN affiliated blog, wrote, "He's pretty awesome. He's cheap and young and he's a Phillie. Ken Giles is one of the reasons we should all feel good about this offseason and the current rebuilding period." Giles finished fourth in the National League Rookie of the Year voting. He finished the season with a 3–1 record, a 1.18 ERA, and one save in 44 games.
