Hartnett was an all-around player, performing well both offensively and defensively. Known for his strong and accurate throwing arm, he routinely led the National League's catchers in caught stealing percentage and was the first major league catcher to hit more than 20 home runs in a season. During the course of his career, Hartnett took part in some of the more memorable events in Major League Baseball history including; Babe Ruth's Called Shot during the 1932 World Series, Carl Hubbell's strike-out performance in the 1934 All-Star Game and Dizzy Dean's career-altering injury during the 1937 All-Star Game. However, the greatest moment of Hartnett's career came with one week left in the 1938 season, when he hit a game-winning home run in the bottom of the ninth inning to put the Cubs in first place. The event, which occurred as darkness descended onto Wrigley Field, became immortalized as the "Homer in the Gloamin"'.