The longtime Williams College professor helped coin two adjectives now common in politics: "transformational" leaders, or those with a vision to change the world, and "transactional" leaders, those with the cunning to get things done.
The words were used constantly during the 2008 presidential race, with the "transactional" Hillary Rodham Clinton battling the "transformational" Barack Obama for the Democratic nomination.
Burns was a liberal Democrat who both wrote about and participated in the political process. He was a convention delegate, congressional aide and congressional candidate who in the late 1950s became friendly enough with Sen. John F. Kennedy to be granted access for a 1960 biography that angered the family by portraying him as a man of excessive calculation and questionable heart.
His two-volume biography of Roosevelt was praised by historians as a model of accessible, objective scholarship. The second volume, "Roosevelt: Soldier of Freedom," was published in 1970 and won the Pulitzer and the National Book Award.
Burns' other books included "Leadership," a 1978 release that outlined his theories of "transformational" and "transactional" and became standard reading among students of business and politics; a biography of George Washington written with Dunn; and a trilogy on U.S. history, "The American Experiment."
In his late 80s, he wrote a well-reviewed history of the Supreme Court, "Packing the Court," and at age 95 he completed a book on the Enlightenment, "Fire and Light."
Burns was born in 1918 in Melrose, Massachusetts, the son of a conservative businessman. He majored in political science at Williams and received a Ph.D. in government from Harvard in 1947, the same year he began teaching at Williams.
He was an Army combat historian during World War II, recording the memories of soldiers just off the battlefield in Okinawa and elsewhere in the Pacific. He earned four combat medals and the Bronze Star.
Later, he worked on a task force headed by Herbert Hoover and served as a congressional aide in Washington, where he was surprised by private scandal (he recalled hearing one drunken legislator brag about his womanizing), and fascinated by how democracy worked.
Burns' first book, "Congress on Trial," came out in 1949 and was praised by The New York Times as a timely assessment of how federal legislators were deadlocked by local concerns.
"Roosevelt: The Lion and the Fox" was published in 1956, at a time when little serious work existed on FDR, who died in 1945.
"I was very interested in how Machiavellian he was," Burns told The Associated Press in 2007. "He was a manipulator, and at the same time he had to be a lion. To what extent did he use the tactics of a fox in order to advance the wishes of a lion? To what extent did he have to be a transactional leader to be able to become a transforming leader?"
The book has dated in some ways — Burns dismissed rumors, since confirmed, that FDR had an affair with his wife's secretary, Lucy Mercer — but "The Lion and the Fox" is still regarded as a landmark.
Burns was the first major biographer to present the president without bias, as an idealist and dealmaker — a gifted, crafty, sometimes inscrutable politician who often kept even his allies guessing what he would do. Roosevelt was also the rare political leader McGregor credited with being both transformational and transactional.
Burns had a more personal and complicated experience as the biographer of a living politician, Kennedy. Burns and Kennedy had gotten to know each other in 1958, when Kennedy was (successfully) seeking re-election to the Senate, and Burns (unsuccessfully) seeking election to the House.
Kennedy worked to boost Catholic support for Burns, while Burns was happy to help Kennedy among Protestants. They got on so well that when Burns agreed to write a biography of Kennedy, who was planning to run for president in 1960. The family promised full cooperation.
In some ways, Burns justified the Kennedys' trust. His book dutifully repeated a cover story about JFK's sister Rosemary, whose mental disability the family wanted hidden from the public. Burns described her as a "sweet, rather withdrawn girl" who cared for "mentally retarded children."
But he also labeled John F. Kennedy "casual as a cash register" and imagined that a Kennedy administration would be "quiet, taut, efficient — sometimes, perhaps, even dull." Jackie Kennedy wrote to him, saying that Burns had "underestimated" her husband.
"The expectation was that it would practically be a campaign tract, which was never my intention, obviously," Burns explained to the AP. "And when the book came out, they were exceedingly disappointed."
Burns was critical of most presidents, finding Bill Clinton too willing to compromise and George W. Bush too partisan.
Burns was married twice and had four children. Dunn said Burns will be buried at Williams College. A private funeral is planned for Saturday.