Ken Moffett, a longtime federal mediator who spent 20 years trying to find solutions to nettlesome labor disputes and then had a brief, unhappy tenure as head of the baseball players’ union, died on Nov. 19 at his home in Alexandria, Va. He was 90.

His death was confirmed by his son Ken Jr., the director of negotiations at the National Treasury Employees Union.

In 1981, Mr. Moffett was the deputy director of the Federal Mediation and Conciliation Service, and faced a summer packed with labor battles involving air traffic controllers, baseball players and postal workers.

“As he shuttled from one bargaining table to the other trying to handle these three national crises, he was under more pressure than any director of the Federal Mediation and Conciliation Service since the agency had been founded in 1947,” Joseph McCartin, author of “Collision Course: Ronald Reagan, the Air Traffic Controllers, and the Strike that Changed America” (2011), wrote in an email.

During the 1981 baseball season, the Major League Baseball Players Association went on strike against team owners over free agent compensation; at one point the players accepted a proposal, with reservations, by Mr. Moffett to end the strike, but the owners spurned it. The strike ended after 50 days at the end of July, partly because of Mr. Moffett’s mediation, about a week after he had helped the United States Postal Service and union workers agree on a new contract.

Despite the agreement, Mr. Moffett recalled the mutual disdain between the players and owners (which is once again being reflected in the owners’ lockout of players that began this month).

“Both sides were very vitriolic toward one another,” he told The Los Angeles Times in 1985. “I brought the secretary of labor” — Raymond Donovan — “in on the fourth day to show them I was serious. It showed them that the secretary of labor and the president were interested in ending the strike.”

But the dispute between the Professional Air Traffic Controllers Organization and the Federal Aviation Administration proved catastrophic for the union, despite Mr. Moffett’s intervention. At one point, he helped broker an agreement — in talks with Drew Lewis, the transportation secretary, and others — that addressed some of the controllers’ demands.

But the union’s membership rejected the agreement, and the controllers went on strike that August. When more than 11,000 controllers did not return to their jobs after 48 hours, President Ronald Reagan fired them, crushing the union, which was soon decertified by the federal government.

Mr. Moffett, who had dealt with the union since 1970, was not surprised by its defiance of Reagan.

“They thought that the government and the country could not exist without them acting,” Mr. Moffett said when Professor McCartin, who teaches history at Georgetown University, interviewed him for his book in 2001. “There are some people that are just not replaceable, per se, and they felt they were these guys.”

Kenneth Elwood Moffett was born on Sept. 11, 1931, in Lykens, Pa., north of Harrisburg. His father, Elwood, was a coal miner who rose to the presidency of District 50, a union of workers in fields related to coal mining that was part of the United Mine Workers and later the United Steelworkers of America. His mother, Hannah (Ely) Moffett, was a homemaker.

Unionism was ingrained in the Moffett family. Ken’s great-great-grandfather was part of the Molly Maguires, a secret society of Irish American coal miners that battled mine owners in the 19th century. His grandfather, who was active in the U.M.W., died of black lung disease. And his father took him to labor meetings and to factories, where Ken handed out pamphlets to unorganized workers.

After graduating from the University of Maryland with a bachelor’s degree in physical education in 1958, Mr. Moffett worked as an organizer for District 50 in Baltimore and Richmond, Va. In 1961, he left for the federal mediation service as an intern. His job involved research; he later moved into mediating disputes in Cleveland for five years, then worked in Washington as a troubleshooter at the agency.

“I get along with people pretty well,” he told The Washington Post in late 1981, after his busy summer. “You can’t have someone doing this who’s acerbic.” He added, “I know all the catchwords and the key jargon which makes people say, ‘Hmm, he is being sympathetic.’”

He became the service’s director of mediation services in 1972, deputy director in 1977 and acting director four years later.

Even with his growing administrative responsibilities, he remained active in mediating disputes, like the pressmen’s strike at The Washington Post in 1975 and the newspaper strike in New York City in 1978 that shuttered The New York Times and The Daily News for nearly three months, and The New York Post for 56 days.

In 1982, President Reagan appointed Mr. Moffett the director of the service, but replaced him after several months with Kay McMurray.

Later that year, Mr. Moffett was unanimously elected executive director of the baseball players’ union, succeeding Marvin Miller, its transformative leader. But he was fired after 11 months on the job. He clashed with Mr. Miller, who had stayed on as a consultant, and he suggested that he had been fired for negotiating a drug-testing policy and for being too conciliatory to owners.

“I was trying to build a bridge to management,” Mr. Moffett told The Washington Post in 1984. “However, the union is determined to be confrontational on every issue. They’re still back on a 1930s tack.”

Mr. Miller responded that Mr. Moffett “comes from a background as a mediator, where any settlement is seen as a victory. No advocate can look at it that way.”

Mr. Moffett moved on to another union, the National Association of Broadcast Employees and Technicians. He was named assistant to the union’s president in 1985 and remained through its merger nine years later with the Communications Workers of America. In the three years before his retirement in 2003, he was the C.W.A.’s human resources director. He continued to work as an independent arbitrator.

In addition to his son Kenneth Jr., Mr. Moffett is survived by his wife, Mary (Taddeo) Moffett; another son, John; a daughter, Laura Tornell; his brothers, Jack and A. Robert; three grandchildren; and three great-grandchildren. His marriage to Barbara Wilcox ended in divorce, as did three subsequent marriages.

Mr. Moffett’s union background and his experience in mediating disputes gave him a healthy respect for collective bargaining, Professor McCartin said. Mr. Moffett’s belief in the process, he added, fueled his stature as “one of the most skilled and dedicated mediators to have emerged in U.S. labor relations in the postwar era.”