Pennock pitched to a 19–6 win–loss record in the 1923 season, his first with the Yankees, leading the American League (AL) in winning percentage () and finishing sixth in wins. Pitching in the 1923 World Series, Pennock defeated the New York Giants in game two, on October 11, to end their eight-game World Series winning streak. He recorded a save in securing the Yankees' win in game four, and pitched to the win in game six on one day of rest, clinching the Yankees' first World Series championship. Umpire Billy Evans called it "the greatest pitching performance I have ever seen", as Pennock "had nothing."