Edmund Morris, a stylish, Pulitzer Prize-winning biographer who wrote a monumental history of Theodore Roosevelt and divided critics with “Dutch,” an authorized biography of Ronald Reagan that featured several fictional characters, died May 24 at a hospital in Danbury, Conn. He was 78.

His wife, fellow biographer Sylvia Jukes Morris, said he suffered a stroke the previous day while at home in the nearby town of Kent.

Raised in British colonial Kenya, the soft-spoken, piano-playing Mr. Morris never received a college degree and was not trained as a historian. But he became a leading practitioner of narrative biography, known for crafting elaborate, cinematic accounts of Beethoven, Roosevelt and Reagan, with a volume on the inventor Thomas Edison scheduled for publication in October.

For Mr. Morris, biography was a great journey into the unknown, or something comparable to a colonoscopy. Interviewing friends and relatives of the late president to nail down the sound of Roosevelt’s laugh, poring over Reagan’s diary in search of insights into his character, the author collected details on a vast supply of 5-by-8-inch cards, then slowly shaped his material into a manuscript. Fixated by the mechanics of his sentences, he said he wrote no more than 300 words a day in longhand with a green fountain pen.

His career as a biographer began in the 1970s, after he turned from advertising to freelance work in New York City. He had written an unproduced screenplay about Roosevelt’s ranching years in the Badlands of North Dakota and embarked on a biography at the encouragement of his agent.

The book was intended as a single volume, but after Mr. Morris traveled to Roosevelt’s home on Long Island, known as Sagamore Hill, his material began to sprawl, ultimately taking the form of a 2,500-page trilogy. Its first volume, “The Rise of Theodore Roosevelt” (1979), won the Pulitzer and National Book Award for biography, and became a best-selling Book of the Month Club selection.

Among its many fans was Ronald Reagan and his advisers, who invited Mr. Morris to serve as the newly elected president’s official biographer. Mr. Morris said he initially found Reagan “bland” and “uninteresting” and dismissed the president’s requests until 1985, when he watched Reagan deliver a moving speech at Bergen-Belsen, the former Nazi concentration camp. “I should be chronicling this,” he recalled thinking.

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Taking a hiatus from his Roosevelt project, Mr. Morris secured a record $3 million advance from Random House and devoted the next 14 years to Reagan, working out of the White House Library and interviewing the president about once a month. (“I’m not going to charge up San Juan Hill for you,” Reagan joked in their first meeting, referring to Roosevelt’s exploit in the Spanish-American War.)

But there was a problem: Reagan, Mr. Morris later recalled, was “simply boring.” The Gipper came to life only when others entered the room. And after more than a decade of research, Mr. Morris found himself with a difficult case of writer’s block.

The solution came to him like a bolt of lightning one day in 1992, as he stepped on an acorn while walking across the campus of Reagan’s alma mater, Eureka College. To place himself at Reagan’s side, he devised a fictional “Edmund Morris” who knew the president from childhood, as well as several other fabricated members of the Morris clan, whose lives were documented in bogus footnotes.

The book featured no printed acknowledgment that it was dabbling in fiction, aside from a brief reference on the jacket flap. Mr. Morris wanted his biography “to weave its own spell” and was convinced readers would quickly realize what he was up to.

“See, what I’m doing is giving flesh to the biographical mind,” he told the Boston Globe. “Instead of saying, ‘Ronald Reagan at 16 stood 6 feet high and wore a wool swimsuit,’ I just simply described it from the point of view of an eyewitness. The reason it worked so well in this book is that his whole life was performance, and performance is meaningless without a witness, without an audience.”

The result, “Dutch: A Memoir of Ronald Reagan” (1999), was a national sensation, spurring debate over the subjective element of biographies and nonfiction more broadly. Peggy Noonan, Reagan’s former speechwriter, called the book a “belly-flop into the pools of Narcissus”; Washington Post columnist George F. Will declared that it was “dishonorable” and “an act of bad faith.”

“Morris’s book is an intellectual embarrassment,” Princeton politics professor Fred Greenstein wrote in a review for Political Science Quarterly. “It blurs fact with fiction, substitutes effusion for rigorous analysis, and is riddled with errors. It also is seriously incomplete, especially in its treatment of the aspect of Reagan’s experience to which Morris might have been expected to have the most to add — his White House years.”

Some critics were far kinder, with Christopher Lehmann-Haupt of the New York Times writing that he could “think of few conventional political biographies that bring their subjects’ pasts so richly alive.”

Mr. Morris, who always insisted that the biography was backed up by research, delighted in the controversy. His aims, he told the Times, were always more literary than journalistic. “I do not think of myself as a historian,” he said. “I’ve always thought of myself only as a writer.”

Arthur Edmund Morris was born in Nairobi on May 27, 1940. His father was a pilot for East African Airways, and his mother was a homemaker. The family moved to South Africa during the Mau Mau rebellion in the 1950s, by which time Mr. Morris had begun writing novels while sitting in the back of the classroom.

He briefly studied at Rhodes University and embarked on a career as an advertising copywriter because it enabled him to write. Settling in London, he developed an informal arrangement with a boardinghouse near his office, where he was allowed to play the piano in exchange for performing chores twice a week.

One of the residents, Sylvia Jukes, walked in on him cleaning the floors one day; they married in 1968 and immigrated to the United States, where they collaborated on travel cassettes for Trans World Airlines, researching the histories and cultures of European countries.

The work was superficial but revelatory. “We realized that we were biographers at heart,” Sylvia Morris said in a phone interview, adding that Mr. Morris turned to Roosevelt as a subject in part because of President Richard M. Nixon’s resignation address, which quoted a few lines from the 26th president.

Mr. Morris concluded his trilogy with “Theodore Rex” (2001) and “Colonel Roosevelt” (2010). His other works included “Beethoven: The Universal Composer” (2005) and “This Living Hand” (2012), an essay collection. He also contributed to publications including the New Yorker, the New York Times and The Washington Post, where he wrote occasional music reviews.

In addition to his wife, who has written biographies of Clare Boothe Luce and first lady Edith Roosevelt, survivors include a brother and sister.

“Biography, in a way, is a strange form of love,” Mr. Morris told the Globe in 1999. “It has all the ingredients of love except it should not have too much affection. But in it one finds the mystery of the initial attraction; the insatiable curiosity, which never flags; the long-term commitment; and the interaction of character. The only ingredients that are missing are adoration and, of course, sexuality.”