NEW YORK — Bob Shane, the last surviving original member of the Kingston Trio, whose smooth close harmonies helped transform folk music from a defiant underground genre into a dominant brand of pop music in the 1950s and ’60s, died on Sunday in Phoenix. He was 85.

Craig Hankenson, his longtime agent, confirmed the death, in a hospice facility.

Mr. Shane, whose whiskey baritone was the group’s most identifiable voice on hits such as “Tom Dooley’’ and “Scotch and Soda,’’ sang lead on most of Kingston Trio songs.

He didn’t just outlast the other original members: Dave Guard, who died in 1991, and Nick Reynolds, who died in 2008; he also eventually took ownership of the group’s name and devoted his life to various incarnations of the trio, from its founding in 1957 to 2004, when a heart attack forced him to stop touring.

Along the way, the trio spearheaded a reinvention of folk as a youthful, mass media phenomenon; at its peak, in 1959, the group put four albums in the Top 10 at the same time. Touring into the 21st century, the group remained a nostalgic presence for its fans, drawing many to its annual Trio Fantasy Camp in Scottsdale, Ariz.

He was born Robert Castle Schoen in Hilo, Hawaii, to Arthur Castle Schoen and Margaret (Schaufelberger). His father, whose German ancestors had settled in Hawaii in the 1890s, was a wholesale distributor of toys and sporting goods. His mother, from Salt Lake City, met her future husband when both were students at Stanford University in the 1920s.

In Hilo, Mr. Shane’s father had planned for Bob to take over the family business. But at the private Punahou School in Honolulu, Bob learned the ukulele and songs of the Polynesian Islands and met Guard, with whom he formed a duet. After high school, Mr. Shane, Reynolds and Guard occasionally played together while attending college in Northern California — Mr. Shane and Reynolds at Menlo College and Guard nearby at Stanford.

After graduating in 1956, Mr. Shane returned to Hawaii to learn the family business, but he found himself more drawn to music. As he told it, he performed as “the first-ever Elvis impersonator’’ and counted Hawaiian music, Hank Williams, Harry Belafonte, and the Weavers as among his influences.

A year later, when Guard and Reynolds decided to make a go of a professional music career, Mr. Shane joined them and returned to California, where the Kingston Trio was born, in 1957. The name, a reference to Kingston, Jamaica, was meant to evoke Calypso music, which was popular then. The members exuded a clean-cut collegiate style, exemplified by their signature look: vertically striped Oxford shirts.

A year later, its first album on Capitol Records included a jaunty version of a ballad based on the 1866 murder of a North Carolina woman and the hanging of a poor former Confederate soldier for the crime. The song, “Tom Dooley,’’ rose to No. 1 on the singles charts, selling 3 million copies and earning the trio a Grammy for best country and western performance. (There was no Grammy category for folk at the time.)

From its founding to 1965, the group had 14 albums in Billboard’s Top 10, five of which reached No. 1. Among their hits was a cover of the whimsical “MTA,’’ about the travails of a harried and nickel-less Charlie who spends the rest of his days on the Boston subway because he does not have money for an exit fee.

The trio inspired scores of imitators and, for a time, was probably the most popular music group in the world.

The Kingston Trio’s critical reception did not match its popular success. To many folk purists, the trio was selling a watered-down mix of folk and pop that commercialized the authentic folk music of countless unknown Appalachian pickers. And mindful of the way that folk musicians like Pete Seeger had been blacklisted during the McCarthy era, others complained that the trio’s upbeat, anodyne brand of folk betrayed the leftist, populist music of pioneers Woody Guthrie and Cisco Houston.

Members of the trio said they had consciously steered clear of political material as a way to maintain mainstream acceptance. Besides, Mr. Shane said, the folk purists were using the wrong yardstick.

“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: 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 Bob Dylan, Phil Ochs, and Joan Baez, and by hipper ones, including Peter Paul and Mary. Then the coming of the British invasion and the rise of rock utterly marginalized the group.

Over time, others, including Dylan and 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.

As Baez wrote in her memoir “And a Voice to Sing With’’: “Before I turned into a snob and learned to look down upon all commercial folk music as bastardized and unholy, I loved the Kingston Trio. When I became one of the leading practitioners of ‘pure folk,’ I still loved them.’’

Mr. Shane’s admirers said his talents were never fully recognized. “Bob Shane was, in my opinion, one of the most underrated singers in American musical history,’’ George Grove, a trio member since 1976, said in an e-mail in 2015. “His voice was the voice, not only of The Kingston Trio but of an era of musical storytelling.’’

The group disbanded in 1967, but after a brief stint as a solo artist, a year later, Mr. Shane was back, first with what was billed as the New Kingston Trio, then with various Kingston Trio lineups.

After retiring, he lived in Phoenix in a home full of Kingston Trio memorabilia. His survivors include his wife, Bobbi (Childress) Shane. He had two children from an earlier marriage, to Louise Brandon. A complete list of survivors was not immediately available.