In 1947, Mr. Hughes graduated from Carleton College with a degree in government relations. He then traveled to Oxford University, where he spent the next two years as a Rhodes scholar.
He remained politically active: Among other endeavors, he went on the BBC in 1948 to defend President Harry Truman, and, that same year, he spent six weeks in Italy working for liberal political campaigns in an election that threatened to see Communists take power.
After returning to the United States and graduating from Yale Law School in 1952, he spent two years in the Air Force as a lawyer.
He married Jean Reiman in 1955; she died in 1993. Along with his second wife, whom he married in 1995, he is survived by his sons, Thomas and Allan, and his sister, Marianne Hughes Nordholm.
Mr. Hughes spent the rest of the 1950s toggling between top jobs with Humphrey and Chester Bowles, who had been governor of Connecticut and, in 1958, won election to Congress. Bowles, who had also served as ambassador to India, was a close adviser to John F. Kennedy’s 1960 presidential campaign, and when he became deputy secretary of state in 1961, he brought Mr. Hughes with him.
He was popular enough among the Washington establishment that, in 1969, the incoming secretary of state, William Rogers, insisted he remain in place at INR. Over the objections of President Richard M. Nixon, Mr. Rogers then promoted Mr. Hughes to be second-in-command at the American embassy in London.
He left government in 1970 and never returned, despite occasional entreaties — President Jimmy Carter twice asked him to run the C.I.A. Though he was as well connected as anyone on the Georgetown cocktail-party circuit, he preferred to keep a low profile behind the scenes.
“Those who talk about power are those who lack it themselves,” he told The New York Times in 1981. “Power corrupts, and lack of power corrupts absolutely.”