John R. Steelman, 99, who became one of President Harry S. Truman's leading aides while holding a succession of executive branch posts that were as important as they were unglamorous, died of pneumonia July 14 at an assisted-living facility in Naples, Fla. He had lived in Florida since the 1960s.

Dr. Steelman came to Washington in 1934 as a member of the U.S. Conciliation Service, then served as service director from 1937 to 1944. He was credited with being the lead figure in settling everything from a Washington hotel strike to labor-management disputes in the aircraft industry. As the public member of a special three-member presidential board, he cast the deciding vote that granted a union shop to the United Mine Workers against U.S. Steel.

In 1945, he went to New York, where he became a management-labor consultant and public relations executive, only to be called back to Washington by the new president, Truman.

Dr. Steelman was named an aide to the secretary of labor and the assistant to the president. He stayed in the White House until 1953. During that time, he played a leading role in settling two national railroad strikes and one of the nation's most serious labor disputes, the confrontation of the steel industry with the United Steelworkers during the Korean War.

Dr. Steelman directed the negotiations that finally resolved that crisis but not before an impetuous president had seized the steel industry under what he called "inherent" presidential emergency powers--an act found unconstitutional by the U.S. Supreme Court.

In addition to his more visible work in labor-management disputes, Dr. Steelman also wore a series of hats as director of several offices and commissions, often simultaneously. He had chaired both the President's Scientific Research Board and the President's Special Commission on Higher Education. He also had been chairman of the National Security Resources Board and served as acting director of the Office of Defense Mobilization.

Dr. Steelman was a leading voice in measures that involved the economy, as well as labor and industrial policies, and the allocation of national resources and manpower. He was not involved in the more visible political affairs. He used to say that in 1948, when Truman hit the whistle-stop trail to campaign for the presidency, he told Dr. Steelman to take care of the White House while he took care of the Republicans.

Truman, who did not seek reelection in 1952, was succeeded by Republican Dwight D. Eisenhower.

Eisenhower asked Dr. Steelman to remain at the White House for about six months to assist in the presidential transition.

After that, Dr. Steelman was a labor-management consultant in Washington and served for a time as publisher of a suburban newspaper company in Silver Spring, before retiring to Florida.

Dr. Steelman, a past president of Record Publishing Co. and the Gardner School of Business, had been a director of several business concerns, including Computer Dynamics Corp. and the American Growth Investment Co.

He was a past president of the Damascus Rotary Club. He had served on the advisory board of American University as well as the boards of Suburban Hospital and the Metropolitan Washington Board of Trade. He was a member of Columbia Country Club, Washington Golf and Country Club, Shady Grove Country Club and the National Press Club.

Dr. Steelman, an Arkansas native, was an Army veteran of World War I. He was a 1922 graduate of Henderson-Brown College in Arkadelphia, Ark., and received both master's and doctoral degrees in sociology from Vanderbilt University. He received a second doctorate, in economics, from the University of North Carolina.

He served as an instructor at Harvard University before becoming a professor of economics and sociology at Alabama College. His work in settling a railroad strike in Alabama drew the attention of Frances Perkins, the new secretary of labor, while she was traveling in the state.

She was so impressed by his work that she brought him to Washington.

His marriages to Jean and Emma Steelman both ended in divorce.

Survivors include his wife of 38 years, Ellen, of Naples, a stepson, Robert Hart of Woodbridge; two brothers; six grandchildren; and five great-grandchildren. An adopted daughter, Doris Frakes, died this year.