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.