Marvin Miller, speaking to reporters after rejecting a proposal to end a baseball strike in July 1981. (Associated Press file photo)

Marvin Miller, the founding executive director of the Major League Baseball Players Association who led a revolution in sports by expanding the concept of free agency, which made many star athletes multimillionaires, died Nov. 27 at his home in Manhattan. He was 95.

His daughter, Susan Miller, confirmed the death to news outlets. He was diagnosed with liver cancer in August.

When Mr. Miller was approached to lead the players union in 1965, the players’ primary goal was to increase their pension fund. He had been the top labor negotiator for United Steelworkers and had served on presidentially appointed labor-management panels. Quiet and urbane, with a matinee-idol mustache, Mr. Miller did not conform to the stereotypical image of a hard-knuckled union organizer.

“I loved baseball, and I loved a good fight, and, in my mind, ballplayers were among the most exploited workers in America” he wrote in his 1991 memoir, “A Whole Different Ball Game.”

Leading the players union seemed initially like a thankless task at best. The union had only $5,400 in its coffers and no staff. Two other candidates had turned down the job before Mr. Miller accepted in 1966.

During his 17 seasons at the helm of the New York-based players association, Mr. Miller forged one of the unlikeliest and strongest unions in the country, battling an entrenched group of wealthy team owners.

He helped establish several landmark advances, most notably free agency and salary arbitration, which gave players a greater voice in determining their salaries and guiding baseball’s future. He also organized four labor stoppages, including a mid-season strike in 1981 that halted the game for two months and alienated many fans previously sympathetic to the players’ cause.

“I think he’s the most important baseball figure of the last 50 years,” former baseball commissioner Fay Vincent told the Associated Press. “He changed not just the sport but the business of the sport permanently, and he truly emancipated the baseball player — and in the process all professional athletes.”

When Mr. Miller took over the players union, the minimum salary was $7,000. Today, it is $480,000. Major league players now earn on average more than $3 million a year.

His other triumphs included the right to collective bargaining, the right to use agents to negotiate contracts, arbitration in labor disputes and the contractual ability of players to block trades to other teams. The model Mr. Miller created for baseball players was later copied by unions in other sports, including the National Football League, the National Basketball Association and the National Hockey League.

In spite of his undeniable achievements in improving conditions for players, Mr. Miller was repeatedly denied a place in the National Baseball Hall of Fame. On five occasions, he failed to gain the required 75 percent of votes of the hall’s veterans committee, which was once dominated by old-school players and executives with a lingering resentment of Mr. Miller’s role in changing the game.

“He is one of the 10 most important figures in baseball history,” sports broadcaster Bob Costas said Tuesday in an interview. “It is regrettable that he’s not in the Hall of Fame. It’s an embarrassment. It’s an absurd omission.”

Fighting the reserve clause

In his early days as union leader, Mr. Miller would meet with players during spring training, traveling from one baseball camp to another in Arizona and Florida.

Owners sought to brand Mr. Miller as a communist, a menace who would destroy the game, and clubs went to interesting lengths to disrupt his meetings. In one instance, a coach hit baseballs into the outfield near where Mr. Miller was talking to players.

Calling the standard player’s contract “one of the worst labor documents” he had ever seen, Mr. Miller negotiated an increase in the minimum salary to $10,000 in 1968. In that first labor agreement, he also secured the right to independent arbitration, which would be hugely important after free agency took effect.

The owners’ control of the financial underpinnings of baseball resided in what was known as the “reserve clause” of a player’s contract. Under this provision, a contract could be automatically renewed year after year. A player was bound to a team forever unless the team decided to trade or release him.

Mr. Miller maintained that the reserve clause was in effect for only a period of one year. If a player refused to sign a new contract and the team automatically renewed the previous year’s agreement, he argued, the player would be bound to the team for only the next baseball season— not in perpetuity.

After fulfilling his contractual obligations for one year, a player could offer his services to other teams. He would become, in effect, a free agent.

One of the few players to challenge the reserve clause was outfielder Curt Flood, who had refused a trade to the Philadelphia Phillies in 1969 after 14 years with the St. Louis Cardinals. In 1972, the U.S. Supreme Court ruled against Flood, largely on grounds that baseball had been declared exempt from antitrust rules in a 1922 high-court decision.

Mr. Miller then devised a strategy to take the teeth out of the reserve clause by recommending that players not sign new contracts. After one year at the previous season’s salary, the clock would start ticking down to free agency.

In 1975, two pitchers — the Los Angeles Dodgers’ Andy Messersmith and the Montreal Expos’ Dave McNally — refused to sign contracts offered by their teams.

Mr. Miller urged them to file grievances, which were heard by an independent arbitrator, Peter Seitz. He ruled that the two players had fulfilled the obligations of their contracts and were, in fact, free agents who could seek employment elsewhere. The baseball establishment challenged the ruling in court but to no avail.

The reserve clause was broken, and free agency became part of a new collective bargaining agreement negotiated by Mr. Miller. Currently, a player with six or more years of major league service who has not signed a contract is eligible for free agency. Contrary to the fears of baseball owners, the game enjoyed a huge boost in attendance, fan interest and skyrocketing television income after free agency began in the 1970s.

“All players — past, present and future — owe a debt of gratitude to Marvin, and his influence transcends baseball,” current players association chief Michael Weiner said in a statement. “Marvin, without question, is largely responsible for ushering in the modern era of sports, which has resulted in tremendous benefits to players, owners and fans of all sports.”

Organizing for labor

Marvin Julian Miller was born April 14, 1917, in the Bronx and was raised in Brooklyn. His father sold women’s clothing and was a labor organizer. His mother was a schoolteacher and an early member of a teachers union.

He graduated in 1938 from New York University with a bachelor’s degree in economics and had early jobs with the Treasury Department and New York City’s welfare department. A shoulder damaged at birth prevented his military service in World War II. He worked instead for the National War Labor Board and later had jobs with the Federal Mediation and Conciliation Service and the International Association of Machinists.

In 1950, he began a 16-year association with United Steelworkers in Pittsburgh, eventually becoming the union’s chief economist and top negotiator. He was appointed to labor-management panels by Presidents John F. Kennedy and Lyndon B. Johnson.

His wife of 69 years, the former Theresa Morgenstern, died in 2009. Survivors include two children, Peter Miller and Susan Miller, and a grandson.

After Mr. Miller retired from the players association in 1982, he remained active in baseball as a consultant and a caustic observer.

To the end, he and his successor at the players union, Don Fehr — now the head of the NHL Players Association — resisted making any concessions to baseball’s owners. Their staunch opposition to random drug testing during baseball’s steroid era contributed to the alienation of many fans, who thought the union had grown as selfish and in­transigent as the ownership it had fought against in the past.

“If Marvin Miller’s position had held,” Costas said Tuesday, “there would be no drug testing. There would be impunity for steroid users, and records would be in question.”

Still, Costas and other observers — including Major League Baseball Commissioner Bud Selig — said that Mr. Miller’s contributions to baseball were unparalleled and that he deserves enshrinement in the Hall of Fame.

In 2010, after he fell one vote short, Mr. Miller said, “It is an amusing anomaly that the Hall of Fame has made me famous by keeping me out.”

Matt Schudel contributed to this report.