Connie Mack signed Pennock to his Philadelphia Athletics in 1912. After using Pennock sparingly, and questioning his competitive drive, Mack sold Pennock to the Boston Red Sox in 1915. After returning from military service in 1919, Pennock became a regular contributor for the Red Sox. The Yankees acquired Pennock from the Red Sox after the 1922 season, and he served as a key member of the pitching staff as the Yankees won four World Series championships during his tenure with the team. After retiring as a player, Pennock served as a coach and farm system director for the Red Sox, and as general manager of the Philadelphia Phillies.