Sy Berger, who transformed a boys’ hobby into a high-stakes pop culture niche as the father of the modern-day baseball trading card, died on Sunday at his home in Rockville Centre, N.Y., on Long Island. He was 91.

His death was announced by his family.

Baseball cards date to the 19th century, but for Mr. Berger, the decade after World War II was the perfect time to revitalize them. The Yankees, the Brooklyn Dodgers and the New York Giants dominated baseball, providing a fertile marketing climate aimed at youngsters in the New York metropolitan area who had been born in the immediate postwar years. And throughout the United States, the arrival of television made it possible for youngsters to watch their baseball heroes in action.

In the 1950s, Mr. Berger turned the Brooklyn-based Topps company into a name synonymous with those pieces of cardboard that children could flip (calling out “front” or “back”), pitch (nearest to a wall wins), trade, or simply admire and store in a shoe box.

Mr. Berger introduced Topps cards in 1951. They came with taffy, rather than chewing gum, because a competitor seemed to have exclusive rights to market baseball cards with gum. But the taffy wound up picking up the flavor of the varnish on the cards.