During an exhibition game against the Chicago White Sox on September 9, 1931, Hartnett was photographed while signing an autograph for gangster, Al Capone. After the photograph was published in newspapers across the United States, Hartnett received a telegram from Baseball Commissioner Kenesaw Mountain Landis instructing him not to have his photograph taken with Capone in the future. Hartnett replied with a telegram to the Commissioner whimsically stating, "OK, but if you don't want me to have my picture taken with Al Capone, you tell him."
In the 1937 All-Star Game, pitcher Dizzy Dean kept shaking off Hartnett's signs for a curve ball resulting in a hit by Joe DiMaggio, a home run by Lou Gehrig and finally, a line drive off the bat of Earl Averill that struck Dean on his toe. Dean had been one of the preeminent pitchers in the National League until the injury to his toe eventually led to the end of his baseball playing career. Hartnett ended the 1937 season with a career-high .354 batting average and, finished second to Joe Medwick in voting for the National League Most Valuable Player Award. His .354 batting average in 1937 was the highest batting average by a major league catcher for 60 years until 1997 when, Mike Piazza posted a .362 average.