Barrow continued to use Pennock regularly after Memorial Day, and Pennock finished the season with a 16–8 win-loss record and a 2.71 ERA in 219 innings pitched. He served as the team's ace pitcher in the 1920 season, but subsequently settled in as the Red Sox' third starter. After the 1922 Red Sox campaign, in which he went 10–17, and had seven wild pitches, leading the AL, the New York Yankees began to negotiate with the Red Sox to acquire Pennock. The Yankees traded Norm McMillan, George Murray, and Camp Skinner to the Red Sox for Pennock that offseason. 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." In the 1924 season, he pitched to a 21–9 win-loss record with a 2.83 ERA while striking out a career-high 101 batters. His win total was second in the AL, behind Walter Johnson, while his ERA was third behind Johnson and Tom Zachary, and he finished fourth in strikeouts behind Johnson, Howard Ehmke, and teammate Bob Shawkey. Pennock's 277 innings pitched and 1.220 walks plus hits per inning pitched (WHIP) ratio led the AL in the 1925 season, while his 2.96 ERA was second-best, behind Stan Coveleski. In the 1926 season, he posted a career-high 23 wins, finishing second in the AL to George Uhle. He again led the AL in WHIP (1.265), and issued the fewest walks per nine innings pitched (1.453). During the pennant race, "The Sporting News" called Pennock the "best left-hander in the majors". Pennock earned the wins in game one and game five of the 1926 World Series. He finished game seven of the series, which the Yankees lost to the St. Louis Cardinals. The Yankees reached the World Series, facing the Pittsburgh Pirates. Pennock pitched a complete game against the Pirates in game three of the 1927 World Series, not allowing a hit until the eighth inning.