On July 20, 1938, "The Big Cat" Johnny Mize hit three home runs against the Giants in the second game of a doubleheader at Sportsman's Park in St. Louis. The three home run performance helped lift the Cardinals to a 7-1 victory over the visiting Giants, with Mize being responsible for five of those runs. Mize homered off of Slick Castleman in the first and the third, then put the exclamation point on things with his third big blast off of Bill Lohrman in the eighth. Mize had hit three home runs seven days before this contest, which came in succession off of Brooklyn's Jim Turner. Unfortunately, Mize came out on the wrong end of that tilt, as the Cardinals lost 10-5 against the Dodgers. A week later he would walk away from a three home run day as a winner. The Cardinals scored seven runs in the first contest also, with Terry Moore leading the way with a three hit performance, that included a home run in what proved to be a 7-2 winner. Mize picked up two RBI's during that game, which gave him seven on the day.
The three home runs on that July day in 1938 made Johnny Mize the first player in the history of Major League Baseball to hit three home runs in a game twice during the same season. "The Big Cat" would accomplish the feat six times during his career, which stands as a major league record. It has since been tied by Sammy Sosa, however, it has not been surpassed. Coincidentally, the only time Mize's team won on a day he hit three dingers came on that day in '38. He went 1-4-1 on those days, which may be astounding, but it also is a testament that it takes more than one man to win a ballgame.
Check out the box scores below
Game 1: http://www.baseball-reference.com/boxes/SLN/SLN193807201.shtml
Game 2 (Mize's three home run game): http://www.baseball-reference.com/boxes/SLN/SLN193807202.shtml