The cause was complications of kidney disease, his family said.

Mr. Lynch masterminded Mr. Dinkins's campaigns for Manhattan borough president and mayor and was, as deputy mayor for intergovernmental relations, his closest adviser.

He was instrumental in bringing the Democratic National Convention to New York in 1992, ran Bill Clinton's presidential campaign in the state that year and later became a vice chairman of the Democratic National Committee.

In recent years, with diabetes and a kidney transplanted from his son, William Lynch III, a political consultant, Mr. Lynch faded from public view. But he remained engaged in the political process on the periphery as a consultant for labor unions, corporations and candidates, including the mayoral campaigns of Fernando Ferrer in 2001 and 2005 and Comptroller John C. Liu this year, and H. Carl McCall's campaign for governor in 2002.

He also worked on John Kerry's presidential campaign in 2004, and was involved in Hillary Rodham Clinton's presidential campaign in 2008.

Early in his political career he was often underestimated because of his disarming aw-shucks demeanor. His scattered style as an administrator — he had a bad reputation for returning phone calls, refused to delegate and once boasted jokingly that "one of my strong suits has been the inability to do long-range planning" — masked his shrewd persistence in forging coalitions and delivering on the commitments that kept them together.

Comparing Mr. Lynch to the Rev. Jesse Jackson, Mr. Dinkins said, "He's a country boy like Jesse is a country preacher."

Once, after the City Council approved a controversial mayoral initiative, Mr. Lynch was asked how he had persuaded council members to adopt a proposal that a month earlier was dead on arrival, but which passed with 10 votes to spare.

"It's not lobbying," he said. "It's education."

In 1989, Mr. Lynch ran Mr. Dinkins's first mayoral campaign from a seedy Times Square building. Mr. Dinkins was elected, just barely, on a promise of racial healing, but race dogged him during his four-year term.

Mr. Lynch played a crucial role in putting together the administration's response to racially charged disturbances in the Crown Heights area of Brooklyn and the Washington Heights area of Manhattan, in promoting calm after the verdict in the Rodney King case threatened to ignite tensions, and in leading the mayor's support of the fight against apartheid in South Africa.

Before Mr. Dinkins lost a re-election challenge in 1993 from Rudolph W. Giuliani, a state report criticized the administration's handling of the 1991 riot in Crown Heights, when bands of young black men attacked Hasidic Jews, fatally stabbing one.

The report concluded that the mayor and the Police Department responded too slowly and too ineptly to quell the violence.

But it also found no evidence to support the accusation that Mr. Dinkins and his police commissioner at the time, Lee P. Brown, deliberately withheld police protection from the Hasidim to allow blacks to "vent their rage" over the episode that precipitated the violence: the death of a black child who was accidentally struck by a car driven by a Hasidic man.

The commission expressed skepticism, though, at Mr. Lynch's testimony that he had no recollection of being warned early on that the violence was out of control. He maintained that City Hall mediators were working independently of the police to end the disturbance.

"My position was always, whether that was good or bad, was that we worked different sides of the street," he said.

William Lynch Jr. was born on July 21, 1941, in the farm country around Mattituck, on the North Fork of eastern Long Island, and grew up there. In high school, he later acknowledged, he performed better at football and basketball than in academic subjects.

After serving in the Air Force, he moved to Harlem in the late 1960s to work for a job training and policy institute at the Graduate Center of the City University of New York.

He demonstrated his political skills for the first time in 1975, when he successfully managed Diane Lacey's insurgent campaign for Democratic district leader in Central Harlem and the Upper West Side.

"I kind of got the bug from then on," he said.

He went on to work on the presidential campaigns of Senator Edward M. Kennedy in 1980 and Mr. Jackson in 1984 and 1988. Other candidates for whom he worked included Representative Major Owens of Brooklyn and State Senator David A. Paterson.

"He impressed me because he was the first really dynamic political person I had met who wanted to be a mechanic, who wanted to be on the front line but who didn't want to be a star," said Mr. Paterson, who later served as governor of New York.

After winning the borough presidency in 1985, Mr. Dinkins named Mr. Lynch his chief of staff. Four years later, Mr. Lynch engineered Mr. Dinkins's mayoral primary campaign, in which he wrested the Democratic nomination from the three-term incumbent, Edward I. Koch.

After leaving government, Mr. Lynch became a vice president at Ronald Perelman's company, MacAndrews & Forbes Holdings, and later opened his own consulting firm, Bill Lynch Associates.

He is survived by his wife, Mary; his son; his daughter, Stacy R. Lynch; and a grandson.

Mr. Lynch was always self-effacing, accepting blame when he lost a campaign or debate and deflecting praise when he won.

"I don't think I'm any genius or guru," he said after the mayoral primary in 1989. "I had a group of people around me who knew what they were doing, and my job was to keep them happy."