“To call the Kingston Trio folk singers was kind of stupid in the first place,” he said. “We never called ourselves folk singers.” He added, “We did folk-oriented material, but we did it amid all kinds of other stuff.”

Indeed, some of Mr. Shane’s finest moments, like the smoky cocktail-hour ballad “Scotch and Soda,” had nothing to do with folk. In 1961, Ervin Drake wrote “It Was a Very Good Year” for Mr. Shane. He sang it with the trio long before Frank Sinatra made it one of his classic recordings.

Still, more than any group of its time, the Kingston Trio captured the youthful optimism of the Kennedy years. The title song of a 1962 album was “The New Frontier,” echoing President John F. Kennedy’s own phrase and alluding to his inaugural address with the lyrics “Let the word go forth from this day on/A new generation has been born.”

About the same time, the trio had an unlikely hit with the kind of material it had avoided: Mr. Seeger’s antiwar song “Where Have All the Flowers Gone?”

But by then the trio was on the verge of being supplanted as the face of folk by a new generation of harder-edged singers like Bob Dylan, Phil Ochs and Joan Baez, and by hipper ones like Peter, Paul and Mary. Then the coming of the British invasion and the rise of rock utterly marginalized the group.

Over time, others, including Mr. Dylan and Ms. Baez, have given the group more credit for popularizing folk music and for serving as a bridge to the more adventurous folk, folk-rock and rock of the 1960s.