(Nicki DeMarco/The Washington Post)

When Gregg Allman began singing at Southern bars and clubs in the mid-1960s with his guitarist brother Duane, his untrained voice was “like a cross between Hank Williams with the croup and James Brown with no lips,” he later quipped.

His vocals bore a rough-edged rasp that Mr. Allman gradually refined into one of the most distinctive sounds in American music: a blend of Tennessee twang, traditional soul and gospel, and a hard-won sense of the blues.

Mr. Allman, 69, who died May 27 at his home in Savannah, Ga., was for decades the frontman of the Allman Brothers Band, a pioneering but conflict-ridden blues-rock collective that modeled its guitar runs on the melodies of Brahms and performed instrumental jams inspired by the improvisational jazz greats Miles Davis and John Coltrane.

His family announced the death on Mr. Allman’s official website but did not provide other details. Mr. Allman had a liver transplant in 2010 and had struggled with an irregular heartbeat, among other health problems in recent years. He recently began canceling concerts.

Mr. Allman released a half-dozen albums as a solo artist but was best known for his work with the Allman Brothers Band, which Duane Allman formed in 1969. The elder Allman, a preternaturally gifted slide guitarist, envisioned a lineup of two drummers and two guitarists, anchored by two Allmans — an outfit capable of doubling the musical possibilities and raw sonic power of a more traditional rock group.

Gregg Allman in the early 1970s. (Michael Ochs Archives/Getty Images/Michael Ochs Archives/Getty Images)

The band featured guitarist Dickey Betts, bassist Berry Oakley, and rock drummer Butch Trucks and his jazz-focused counterpart, Jai Johanny “Jaimoe” Johanson. Mr. Allman shared singing duties with Betts, played the electric organ and wrote some of the band’s most enduring songs, including tracks about a road that “goes on forever” (“Midnight Rider”) and “bearing sorrow, havin’ fun” (“Melissa”).

His nimble work on the Hammond B3 organ helped the Allman Brothers become a national phenomenon in the early 1970s, when their improv-heavy concerts packed stadiums and often lasted more than three hours.

Before performances at the Fillmore East, a frequent stomping ground for the band in Manhattan’s East Village, club owner Bill Graham was known to tell audiences that he was locking the auditorium doors — and that anyone who needed to leave during the night should do so before the rocking commenced.

When it did, the music was often revelatory, betraying the brothers’ wide-ranging musical roots. Born in Nashville, they grew up hearing country music on weekend trips to the Grand Ole Opry, and later fell in love with the soul of Ray Charles and the blues of Little Milton when their mother moved the family to Daytona Beach, Fla.

Their eclectic tastes resulted in a new sound, now labeled Southern rock, that crossed genres and in some cases racial divides. Blues was seen as strictly African American music; country, and to some extent rock-and-roll, were taken to be exclusively white. The brothers drew from both worlds and turned up the amps.

Early records featured blues standards alongside original compositions by Mr. Allman, and in 1971 the group broke through critically and commercially with “At Fillmore East.” Considered one of the greatest live albums in rock history for its high-octane improvisations, the record featured extended versions of songs such as “Whipping Post,” stretching the number well past its five-minute studio version into a 23-minute epic.

Within six months of the record’s release, Duane Allman was dead at 24, killed in a motorcycle accident near a Tudor mansion that served as the band’s home in Macon, Ga.

Mr. Allman was devastated. His brother had been his musical lodestar as well as his partner in drug-fueled mischief: As teenagers, the guitarist had hopped up his brother Gregg on alcohol and speed for a “foot-shooting party,” resulting in a well-targeted gunshot wound that kept him out of military service in Vietnam.

By his account, Mr. Allman sank deeper into alcohol and drug abuse following the death of his brother, moving from the psychedelic mushrooms that the band had used as a songwriting aid to ever-increasing quantities of cocaine and heroin.

The band performed more than 300 shows a year on the strength of its first top-10 record, “Eat a Peach” (1972), crisscrossing the country on a private Boeing 720 known as the Starship. When they first climbed aboard, Mr. Allman recalled in his memoir, “My Cross to Bear” (2014), they found the words “Welcome Allman Bros” written in cocaine on the bar.

The bewhiskered Mr. Allman also developed a nickname, Coyotus Maximus, that reflected his seemingly insatiable appetite for what he later described as “foxy ladies . . . oodles of them.”

When the band’s bassist, Oakley, died one year after Duane Allman and in nearly identical circumstances, the band regrouped once again. Their album “Brothers and Sisters” (1973) sold several million copies and featured the band’s best-known song, “Ramblin’ Man,” written and sung by Betts.

The guitarist pushed the band in a more conventionally country direction, and Mr. Allman responded the same year with “Laid Back,” his solo debut and most critically acclaimed record. The album marked a return to the downtempo soul arrangements that Mr. Allman had grown up striving to imitate.

It also marked a slow-building separation between Mr. Allman and the group that carried his name, which eventually broke up and reformed — twice — over creative disputes and, at least in part, animosity toward Mr. Allman.

Band members accused him of leading too glamorous a lifestyle. He married the pop singer Cher in 1975, moved with her to Beverly Hills, and collaborated with her on a disastrously received pop record under the billing Allman and Woman before divorcing. Bandmates also charged that Mr. Allman looked out for himself before the group. When Mr. Allman agreed to testify against his road manager in exchange for immunity in a 1976 drug case, they forswore any future collaboration.

Yet Mr. Allman proved persistent in smoothing things over. He stopped doing drugs (at least for a time), returned to Macon and reunited with Betts, Trucks and Jaimoe to release “Enlightened Rogues” (1979), with the additions of Dan Toler on guitar and David Goldflies on bass. The record took its name from a term Duane Allman once used to describe the band’s original members.

Mr. Allman won special praise for his vocal work on the album, which Rolling Stone writer Robert Palmer described as the finest of his career — “growling, urgent, full of fire.” Experience had made him a better singer, Mr. Allman told the magazine.

“Plus,” Mr. Allman added, “getting away from the, ahhh, depressants — that also helps greatly.”

Gregory LeNoir Allman was born in Nashville on Dec. 8, 1947. His father was fatally shot by a hitchhiker when Gregg was 2. His mother later supported the family by doing accounting work.

At 13, Gregg bought a guitar from Sears and taught himself to play while listening to a jazz-and-blues station. His brother Duane proved more adept at the instrument. “He passed me up inside of two weeks,” Mr. Allman told The Washington Post in 1989 — and Gregg began focusing on the keyboard.

The duo formed a blues-rock group called the Allman Joys and made an unsuccessful recording debut (as the Hour Glass) before forming the group that became the Allman Brothers Band.

The band was inducted into the Rock and Roll Hall of Fame in 1995 and continued playing for nearly a decade more, with several lineup changes in the wake of Betts’s exit from the band in 2000. In Mr. Allman’s memoir, co-written with Alan Light, he said that he was drunk at the hall of fame ceremony and cited the induction as the turning point at which he quit drinking after 14 previous trips to rehab.

He had six marriages and five children: Michael Allman, a Southern rock singer, from a relationship with Mary Lynn Sutton; Devon Allman, leader of the blues-rock band Honeytribe, from a marriage to Shelley Kay Jefts; Elijah Blue Allman, a singer and guitarist for the heavy-metal band Deadsy, from his marriage to Cher; Delilah Island Allman, from a marriage to Julie Bindas; and Layla Brooklyn Allman, who fronts the rock band Picture Me Broken, from a relationship with Shelby Blackburn.

A complete list of survivors was not immediately available.

Although Mr. Allman rarely strayed far from the stage, he said there were times when he felt like quitting.

“You’ve got to consider why anybody wants to become a musician anyway,” Mr. Allman told Rolling Stone in 1973, following his brother’s death. “I played for peace of mind.”