Eddie Adcock, the virtuoso banjo and guitar player who served as a bridge between the formative early years of bluegrass and the innovative “newgrass” movement of the 1970s and beyond, died on March 19 in Lebanon, Tenn. He was 86.

His death, in a hospital, was confirmed by Dan Hays, a former executive director of the International Bluegrass Music Association, who said Mr. Adcock had a number of chronic health problems.

Mr. Adcock brought his improvisatory fretwork to musical settings ranging from the first-generation traditionalism of Bill Monroe to the newgrass, or “new acoustic,” sounds fashioned by forerunners of modern bluegrass like the Country Gentlemen and II Generation.

Mr. Adcock was best known for his tenure in the 1960s with the Country Gentlemen, a group based in Arlington, Va., that, through advances in style and repertoire, all but redefined bluegrass music. Employing a traditional string-band format, they broadened the genre’s appeal with their impromptu arrangements of folk and pop songs and material written by artists like Hedy West and Gordon Lightfoot whose work fell outside the bounds of bluegrass.

Mr. Adcock’s contributions were consistently among the quartet’s most daring, notably his dazzling string-bending and his use of the thumb-style guitar technique of Merle Travis to create a unique jazz- and blues-inflected approach to playing the banjo.

“I released all my insides, all my creativity, into the band,” Mr. Adcock said of his heady early years with the Country Gentlemen in a 2016 interview with Scottsville Monthly, the magazine of his hometown, Scottsville, Va. “I was ready to say something of my own, and that’s where I made my mark.”

The Country Gentlemen predated Mr. Adcock, but the iteration that included him, along with Charlie Waller on guitar, John Duffey on mandolin and Tom Gray on upright bass, was heralded widely as the group’s first “classic” lineup. All of them sang, with Mr. Adcock handling the baritone harmonies in the quartet’s rich interweaving of voices. (Mr. Duffey died in 1996, Mr. Waller in 2004.)

The group inspired a new generation of musicians, including David Grisman, Tony Rice and the group New Grass Revival. Today’s jam-band scene also owes an artistic debt to the Gentlemen, especially to Mr. Adcock’s pyrotechnic syncopation.

“His earliest work alone with the Country Gentlemen concreted his mark on the evolution of bluegrass,” Missy Raines, who played bass with Mr. Adcock from 1985 to 1993, wrote in an email. “Not just because of his distinctive banjo style and his unmatched baritone voice (which blended with Duffey and Waller in an unearthly way), but as much because of his guidance within the band to learn songs from outside the bluegrass genre. He was a visionary.”

Edward Windsor Adcock was born on June 21, 1938, one of seven children of Bennie and Senora Ann (Johnson) Adcock. Daily chores on the family farm frequently interfered with the schooling of Eddie and his siblings, each of whom played a musical instrument and sang. Their father died when Eddie was an adolescent, leaving their mother to seek employment outside the home, including work as a custodian, to supplement the family’s income.

Eddie’s first professional engagement as a banjoist was on local radio when he was 15. Two years later he took a job with Mac Wiseman and the Country Boys; he joined Bill Monroe and the Blue Grass Boys in 1958. Because the pay was insufficient, he also worked a series of day jobs, including auto mechanic and truck driver.

In 1958, Mr. Waller and Mr. Duffey invited Mr. Adcock to enlist in the Country Gentlemen, then a new band. He would go on to play on signature recordings like “Two Little Boys,” “New Freedom Bell” and “This Morning at Nine.”

In 1970, after more than a decade with the quartet, Mr. Adcock moved to California and formed a country-rock band called the Clinton Special. A year later he moved back east and founded the newgrass ensemble II Generation with the mandolinist Jimmy Gaudreau. In 1973 he met Martha Hearon, who would soon join the band as its rhythm guitarist.

The couple married three years later and subsequently performed together as a duo, Eddie and Martha Adcock, and in other formats both onstage and in the studio.

In 1989 Mr. Adcock took part in the bluegrass supergroup recording “The Masters,” alongside Jesse McReynolds on mandolin, Uncle Josh Graves on Dobro and Kenny Baker on fiddle. In 1996 he was inducted into the Bluegrass Music Hall of Fame and Museum as a member of the Country Gentlemen.

He remained active until 2008, when a tremor in his right hand cost him the ability to play. To restore the use of his hand, surgeons at Vanderbilt University Medical Center in Nashville performed deep brain stimulation, a procedure in which an electrode is inserted into the patient’s brain and connected to an electrical device.

Mr. Adcock remained awake throughout the surgery, allowing him to play the banjo to test the effectiveness of his doctors’ interventions. The operation was repeated two more times and ultimately succeeded.

In 2014 he was presented with the fifth annual Steve Martin Banjo Prize, which included a $50,000 award.

In addition to his wife of almost 50 years, Mr. Adcock is survived by his three children, Edward Windsor Adcock Jr., Beatrice Adcock and Dennis Adcock, all from his marriage to Mildred Gorham; four grandchildren; five great-grandchildren; and five great-great-grandchildren. His six siblings died before him. Dennis Adcock played bass with his father’s band from 1979 to 1983.

When discussing his influences, Mr. Adcock did not always cite trailblazing banjo players like Earl Scruggs and Don Reno. But he never neglected to name Pauline Mayo, the first-grade teacher who taught him how to read and write music.

“Expanding my musical world beyond the hillbilly sounds of local groups, and even the Scottsville Orchestra, Mrs. Mayo got me wrapped up tight in all forms of music — opera, jazz, country,” he told Scottsville Monthly. “I’ve given her credit for that in every interview.”