Richardson died Friday of a cerebral hemorrhage at Massachusetts General Hospital after being admitted there on Wednesday, hospital spokeswoman Nicole Gustin said.
In a wide-ranging career, Richardson served as secretary of defense under Nixon, ambassador to Great Britain and U.S. representative to the Law of the Sea Conference during the Gerald Ford administration. He also ran for a U.S. Senate seat in Massachusetts in 1984.
But he was best known for his actions in 1973, when, during the height of the investigation into the break-in at Democratic Party headquarters at the Watergate Hotel, he refused Nixon's orders to fire the special prosecutor in the case, Archibald Cox.
The Republican president was battling Cox over his attempts to subpoena tape recordings of White House discussions believed relevant to the investigation of the Watergate break-in and the suspected cover-up by the Republican president and his staff.
Nixon, who eventually was driven from office by the Watergate affair, contended the nine tapes being sought were privileged.
"The more I thought about it, the clearer it seemed to me that public confidence in the investigation would depend on its being independent not only in fact but in appearance," Richardson wrote in his 1996 book, Reflections of a Radical Moderate.
Cox eventually was fired by Acting Attorney General Robert Bork, whose nomination for the Supreme Court years later would be denied.
Richardson remained a man who followed his instincts, rather than party lines, throughout his life.
In 1994, Richardson, who was a Republican, added his name to the list of trustees of President Clinton's legal defense fund set up to help defray Clinton's legal costs in fighting Paula Jones' sexual harassment allegations and Whitewater charges.
Descended from early New England settlers, Richardson was born in Boston and was related to many of Boston's prominent families.
He earned a bachelor's degree cum laude from Harvard University in 1941, served in World War II, and in 1945 became editor and president of the Harvard Law Review.
After graduation, Richardson practiced law before being appointed U.S. attorney for Massachusetts from 1959 to 1961.
Richardson, who was a partner in the prominent Boston law firm of Ropes and Gray, served as lieutenant governor of Massachusetts from 1965 to 1967.
His services won him the Presidential Medal of Freedom, the nation's highest civilian honor, in 1998.
Richardson retired from the Washington, D.C., law office of New York's Milbank, Tweed, Hadley and McCloy in October 1992.
He married Anne Francis Hazard in 1952, and they had three children.
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