Debuting for the Philadelphia Phillies at the tender age of 18 in 1912, Pennock's early years were marked by inconsistency. Yet, his raw talent was undeniable. He showcased a deceptive fastball, a sharp curveball, and a masterful changeup, keeping hitters off balance. By 1914, he had established himself as a reliable starter, and in 1915, he led the National League with 27 wins, a feat he would repeat two years later.

He spent his prime years with the Philadelphia Athletics and Boston Red Sox, consistently putting up impressive numbers. He led the league in ERA twice and won over 20 games in three seasons, showcasing his mastery over his new pitching style. His calm demeanor under pressure made him a go-to starter in crucial games, earning him the nickname "The Nerves of Steel."

Pennock's career wasn't without its challenges. He was traded multiple times, facing new teams and environments, but his adaptability and dedication to his craft remained constant. In 1931, at the age of 34, he defied expectations by leading the New York Yankees to a World Series victory, cementing his legacy as a big-game pitcher.