Bill Moyers, who served as chief spokesman for President Lyndon B. Johnson during the American military buildup in Vietnam and then went on to a long and celebrated career as a broadcast journalist, returning repeatedly to the subject of the corruption of American democracy by money and power, died on Thursday in Manhattan. He was 91.

His son William Cope Moyers confirmed the death, at Memorial Sloan Kettering Cancer Center. He lived in Manhattan.

To Americans who grew up after the 1960s, Mr. Moyers was known above all as an unusual breed of television correspondent and commentator. He was once described by Peter J. Boyer, the journalist and author, as “a rare and powerful voice, a kind of secular evangelist.”

But before that, Mr. Moyers was President Johnson’s closest aide. Present on Air Force One in Dallas when Johnson took the oath of office after the assassination of President John F. Kennedy, Mr. Moyers played a pivotal role in the inception of Johnson’s Great Society programs, and was the president’s top administrative assistant and press secretary when Johnson sent hundreds of thousands of troops to fight in the Vietnam War.

Mr. Moyers resigned from the administration in December 1966 at age 32, finalizing an irreparable falling out between the hot-tempered, flamboyant Johnson, who demanded unwavering loyalty, and the cool, self-contained Mr. Moyers, whom Johnson had denied several foreign policy positions. The two men never reconciled. In his 1971 memoir, “The Vantage Point: Perspectives of the Presidency, 1963-1969,” Johnson mentioned Mr. Moyers only fleetingly, reducing him to little more than a footnote.