John Gary Williams, whose pleading lead vocals were key to the success of the Mad Lads, a doo-wop-influenced Stax group out of Booker T. Washington High School that recorded several influential and beloved R&B hits in the 1960s, has died. He was 73.

Proudly and fiercely independent, Mr. Williams led an often tumultuous life that was notable for more than music.

Mr. Williams' Stax career was interrupted by military service in Vietnam, where he participated in dangerous deep-jungle combat missions. Returning home, he joined the Invaders, a so-called "militant" group inspired by the Black Panthers that was especially active during the sanitation strike that brought Dr. Martin Luther King Jr. to Memphis, where the civil rights leader was murdered in 1968.

As the singer himself put it, in the title of a 1973 single he recorded for Stax: "The Whole Damn World Is Going Crazy."

Mr. Williams was found dead Tuesday morning at his home, not too far from the Soulsville neighborhood where the Stax Museum of American Soul Music is located. He had been in poor health for some time, and lost his voice following an operation last year for throat cancer.

"The irony of a singer losing his voice can't be underestimated," said journalist and former Memphian John Hubbell, who is finishing a documentary feature film on Mr. Williams. "He couldn't sing, he couldn't talk."

Consisting of Williams, Julius E. Green, William Brown and Robert Phillips (all now deceased), the Mad Lads represented an early attempt by Stax to make inroads on the East Coast, where male vocal harmony groups were especially popular. Following a 1964 novelty single ("The Sidewalk Surf"/"Surf Jerk"), the group had back-to-back R&B hits in 1965 and 1966 with "Don't Have to Shop Around" and "I Want Someone," released on the Volt label, an imprint of Stax.

Mr. Williams' supple lead tenor — a coaxing, insinuating, sometimes sorrowful instrument — was central to the appeal of the group, which specialized in love songs that alternated between hope and heartbreak: "I Want a Girl," "Tear-Maker," "I Don't Want to Lose Your Love."

"The Mad Lads were a huge anomaly at Stax, because they sounded like a Northern soul group," said Memphis musician Scott Bomar, who produced Mr. Williams' final recordings, and whose band, the Bo-Keys, performed with Mr. Williams many times starting in about 2004. "They were huge in Philly and Chicago, and are still huge on the West Coast-East L.A. 'low rider' scene."

Unfortunately, Mr. Williams' shot at nationwide stardom was interrupted when he and Brown were drafted into the military in 1966. Mr. Williams' service was especially dangerous; he served in a long-range reconnaissance patrol unit (commonly known as a LRRP, pronounced "lurp"), taking part in missions that penetrated deep into enemy territory.

After returning from Vietnam in 1968, Mr. Williams rejoined the Mad Lads, and also recorded a self-titled solo album for Stax that was released in 1973, just as the company was going under. Mr. Williams wrote or co-wrote seven of the 10 tracks on the album, which has been reissued on CD and vinyl in recent years and been praised for its mix of "gently percolating grooves and soaring strings" with lyrics about "the backstabbing state of the modern world" (to quote the Allmusic.com website).

The Mad Lads reunited in 1984 and continued to record and perform off and on for many years afterward, in various configurations.

In high school, the group originally had called itself "The Emeralds," but Stax wanted the vocalists to share a more novel and marketable name, according to Deanie Parker, a Stax recording artist and publicist who later was key to the founding of the Stax Museum.

"We named them the Mad Lads because that's exactly what they were — just four mad guys, just mischievous, fun-loving, typical young men," Parker said.

"We were pranksters," Mr. Williams told author Rob Bowman, as quoted in Bowman's book, "Soulsville U.S.A.: The Story of Stax Records." "We brought that high-school thing, that young thing into the company."

Parker and Stax co-founder Estelle Axton co-wrote one of the Mad Lads' first significant Stax recordings, "I Want Someone," using lyrics pulled from what Parker called Axton's hand-written notebook of original lyrics, which Axton called her "book of poems."

"I taught the kids the song, helped to arrange it, I even played piano on the session," Parker said. The song and such followups as "Patch My Heart" and a 1969 version of "By the Time I Get to Phoenix" were R&B hits, but didn't make much impact on the Billboard Pop charts.

According to Hubbell, Mr. Williams' Vietnam experience made him keenly aware of the inequities of American society in general and Memphis in particular.

Mr. Williams became "impatient" and "was not satisfied with the pace of change" in America, said Hubbell (a former reporter with The Commercial Appeal). He said Mr. Williams' involvement in the Invaders — a community-organizing group characterized as "militant" by white and even some black establishment leaders — was motivated by his desire to actively improve conditions in struggling African-American neighborhoods.

"He connected that soul music world with Vietnam and the experience of so many African-American guys who were drafted at the peak of their life and saw their lives forever altered by war and violence," said Hubbell, whose documentary, titled "World Gone Crazy: The Trials of John Gary Williams," is set for release next year.

The "Trials" reference in that title is not just a metaphor. In 1968, Mr. Williams was sentenced to two years in prison after being convicted of "intent to commit voluntary manslaughter" for his disputed role in the non-fatal shooting of a Memphis police officer. (Mr. Williams was on the scene, but was not the shooter.)

In addition, Mr. Williams struggled for a time with drug and alcohol addictions, and later sought therapy for post-traumatic stress disorder, in connection with Vietnam.

After the collapse of Stax, Mr. Williams became something of a wanderer, working in Iowa and Los Angeles (as a taxi driver) before returning to Memphis, while continuing to write songs. For a time, he ran a bar in South Memphis.

Whatever the context, he remained outspoken and uninterested in compromising his values, even when keeping silent might have been to his career benefit. During the 2014 local premiere at the Malco Ridgeway Four of writer-director Tim Sutton's "Memphis," an arty independent drama that floats through the peripheries of the Memphis music scene, a visibly angry Mr. Williams, who appears in the film, stood up and announced: "Worst movie I've ever seen in my life."

Although Mr. Williams is gone, an album's worth of solo material recorded by Mr. Williams just before the Stax bankruptcy remains in the vault, and is being readied for future release, Hubbell said. Also in the wings are Mr. Williams' final two songs as both a recording artist and composer, "A Natural Kind of Thing" and "My Kind of Lady," produced by Bomar at his Electraphonic Recording studio. The songs will be issued as a 45rpm single in conjunction with the release of the documentary.

Said Parker, who helped launch Mr. Williams' career: "He was a very, very interesting young man. As I look back on his life, I would describe it as filled with tragedy."

Mr. Williams leaves his wife, Trenni Williams; five daughters, Alisa Williams, N'Kenge Williams, Tonya Duncan, Angenita Montgomery and Dana Williams; three step-daughters, Kenosha McClendon, Kearston Phillips and Ebonique Cox; two sons, John Gary Williams Jr. and Marvin Phillips; two brothers, Victor Williams and Richard Williams; 11 grandchildren; and six great-grandchildren.