Marvin Miller, an economist and labor leader who became one of the most important figures in baseball history by building the major league players union into a force that revolutionized the game and ultimately transformed all of professional sports, died on Tuesday at his home in Manhattan. He was 95.

His death was announced by the Major League Baseball Players Association. He had liver cancer, his daughter, Susan Miller, said.

When Mr. Miller was named the executive director of the association in 1966, club owners ruled much as they had since the 19th century. The reserve clause bound players to their teams for as long as the owners wanted them, leaving them with little bargaining power. Come contract time, a player could expect an ultimatum but not much more. The minimum salary was $6,000 and had barely budged for two decades. The average salary was $19,000. The pension plan was feeble, and player grievances could be heard only by the commissioner, who worked for the owners....