Mansfield had undergone surgery Sept. 7 to have a pacemaker implanted in his chest after complaining to friends of feeling weak. He died in Walter Reed Army Hospital at 7:35 a.m. EDT, said Charles Ferris, who was Mansfield's attorney and had served as his counsel when Mansfield led the Senate.
Mansfield served 34 years in Congress, including 15 as leader of the Senate's Democratic majority, from 1961 until his retirement from the Senate in 1976. Liberal, gentle, scholarly and expert in Asian affairs, he went on to become U.S. ambassador to Japan, serving two presidents.
At a time when the Senate echoed with strident voices and grew crowded with mounting egos, Mansfield led a quiet revolution of style and substance. He mended a Democratic majority that had been torn by suspicion and mistrust.
His patience cultivated a reservoir of trust in both parties that allowed him to steer the Senate through the tumultuous years of civil rights demonstrations and Vietnam, of Lyndon Johnson's Great Society and Richard Nixon's downfall in Watergate.
He spoke rarely, but chose his words with care and thought. His news conferences were peppered with "yeps" "nopes" and "can't says."
He was his most vocal in his early, fervent and unsuccessful opposition to the Vietnam War, which he called a "grotesque mistake."
In a 1973 interview about the war, Mansfield said: "We used it, I think, as a testing grounds of sorts. We paid a terrible price in dead. ... We wasted our treasure, at least $130 billion. We destroyed towns and cities to save them. We decimated populations and lands through defoliation. ... We have no one to blame but ourselves."
Upon retirement, Mansfield said his greatest disappointment during his more than three decades in Congress was: "I was not able to stop or slow down the Vietnam War."
Mansfield was elected to the House in 1943. He served five terms there before he was elected to the Senate in 1952.
In 1957, he was elected assistant Democratic leader under Johnson, the Texas Democrat who controlled the Senate with an arm-twisting style. When Johnson became vice president, Mansfield became majority leader and orchestrated a change in how the Senate worked.
Born in New York City's Greenwich Village to Irish Catholic immigrants, Mansfield was sent off to Montana to live with relatives after his mother died when he was 3.
He left Montana at 14, lying about his age in order to join the Navy and fight in World War I. He went on to serve in the Army and Marine Corps before returning to Montana in 1922, where he went to work in a copper mine.
It was not until 1927, at the urging of his future wife, Butte, Mont., schoolteacher Maureen Hayes, that he enrolled in the Montana School of Mines.
"She put some sense into me, told me to go to school and make somthing of myself, and I did," Mansfield said in 1970. "I never thought I could get a college education because I left school ran away before I finished the eighth grade."
He graduated from Montana State University at Missoula in 1933 at the age of 30, then went on to obtain a master's degree and to become a professor, teaching history of the Far East and Latin America.
His interest in the Far East stemmed from his travels across China as a Marine Corps private from 1920 to 1922. Mansfield returned to China in 1972 and again in 1974 to meet with leaders of the Peoples' Republic of China.
President Carter appointed him ambassador to Japan in 1977. When the White House changed parties in 1980, President Reagan asked Mansfield to stay on.
Mansfield retired as ambassador in 1988, but continued to work in Washington as a Far East consultant for the New York investment banking firm of Goldman Sachs and Co.
His wife, Maureen, died last year in a nursing home.
©MMI The Associated Press. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed