Fred Hellerman, right, performs at a 1980 reunion concert with the Weavers, including, from left, Pete Seeger, Lee Hays and Ronnie Gilbert. (Richard Drew/AP)

Fred Hellerman, a self-taught guitarist who sang of — and in — harmony with Pete Seeger as a founding member of the pivotal 1950s folk quartet the Weavers, died Sept. 1 at his home in Weston, Conn. He was 89.

He had heart, lung and other ailments, said his son Caleb Hellerman.

Formed in 1948, the Weavers sold millions of records and influenced acts including the Kingston Trio; Peter, Paul and Mary; Joan Baez; and Bob Dylan. (Mr. Hellerman, who in the late ’50s and ’60s began working largely as a songwriter and producer, played guitar on Baez’s self-titled 1960 debut album. Of Dylan, he once said: “He can’t sing, and he can barely play, and he doesn’t know much about music at all.”)

The Weavers’ blend of politically minded lyrics and sunny harmonies reached mass audiences in songs such as “Goodnight, Irene,” “On Top of Old Smoky,” “Kisses Sweeter Than Wine” and “The Hammer Song,” which became a top-10 hit when Peter, Paul and Mary recorded it under the name “If I Had a Hammer” in 1962.

In addition to Seeger and Mr. Hellerman, the Weavers featured Lee Hays, who sang bass, and contralto Ronnie Gilbert. Hays died in 1981, Seeger in 2014, and Gilbert in 2015, leaving Mr. Hellerman the last surviving member of the group’s original lineup.

The Weavers, whose name — suggested by Mr. Hellerman — was taken from a Gerhart Hauptmann play about a 19th-century workers uprising, suffered in the 1950s, when Sen. Joseph R. McCarthy (R-Wis.) and the House Un-American Activities Committee investigated purported communist activity in government and the arts.

In 1950, Seeger was labeled a communist by the publication Red Channels. Two years later, a paid FBI informant testified before HUAC that Mr. Hellerman and Gilbert also were communists. The informant, Harvey Matusow, was later discredited and served 44 months in prison for perjury.

Television appearances were canceled, the American Legion picketed a performance at the Strand Theatre in New York, and on at least one occasion, the Knights of Columbus threatened to shut down a club that booked the folk outfit.

Blacklisted, the group stopped performing but in 1955 reunited for a packed concert at Carnegie Hall in New York. The Weavers had originally planned to hold their reunion show in the smaller, less prestigious Town Hall, Mr. Hellerman later told the New York Times, but that venue refused to let them play. At Carnegie, he added, “we found to our surprise that the act of coming to a Weavers concert was taken as ‘making a statement.’ It carried a sense of defiance.”

The Weavers continued performing together — although without Seeger, who left in the late ’50s to pursue a solo career and was replaced by Erik Darling — until calling it quits after a 1963 concert. They reunited once more at Carnegie Hall in 1980, in a pair of shows captured in the documentary “The Weavers: Wasn’t That a Time!” (1982).

Fred Hellerman — who joked that his family was “too poor to afford a middle name” — was born in Brooklyn on May 13, 1927. His father, a Latvian immigrant, ran a grocery store before selling used fabric.

Mr. Hellerman taught himself guitar while serving on a Coast Guard weather ship during World War II. A fan of jazz and vaudeville music at a young age, he had played bit parts in New York’s Yiddish theater scene before turning to folk music in college, according to his son.

The Weavers emerged from an ensemble called the Almanac Singers, which at various times included Seeger and Hays along with folk musician Woody Guthrie and singers Millard Lampell and Burl Ives.

The group dissolved during World War II, and Seeger and Hays subsequently tried to organize a folk chorus that could spotlight the music of artists such as Huddie Ledbetter, a New York blues guitarist and singer who went by the name Lead Belly.

Mr. Hellerman and Gilbert — who met while working as counselors at a leftist New Jersey summer camp called Wo-Chi-Ca (Workers Children’s Camp) — soon joined the fledgling chorus, which was whittled down to a quartet when, as Gilbert later told the Times, “it became clear that we had an extraordinary blend of voices.”

But the group struggled to gain renown. Mr. Hellerman — who graduated from Brooklyn College in 1949 — was planning to leave New York to pursue a graduate degree in English when the Weavers finally broke through at the Village Vanguard, a New York club that had previously hosted Lead Belly and other folk singers. They were soon signed to Decca Records.

Their first record, a single featuring the Hebrew song “Tzena, Tzena, Tzena” and a version of Lead Belly’s “Goodnight, Irene,” sold 2 million copies.

In addition to his work with the Weavers, Mr. Hellerman was a frequent collaborator with Harry Belafonte, for whom he co-wrote the 1963 hit “Come Away Melinda” with Fran Minkoff. He also produced Arlo Guthrie’s hit debut, “Alice’s Restaurant” (1967), and follow-up live album “Arlo” (1968).

His marriage in 1970 to Susan Lardner, a writer for the New Yorker magazine, ended in divorce, but they later reconciled and lived in Weston at the time of Mr. Hellerman’s death. Besides Lardner, survivors include two sons, Simeon Hellerman of Tokyo and Caleb Hellerman of Framingham, Mass.; and three grandchildren.

In recent years, Mr. Hellerman pushed himself to try new genres as an arranger and composer. Self-conscious about being a self-taught musician, “he wanted to be seen as a serious musician and composer,” his son said.

For his first and only solo record, “Caught in the Act,” Mr. Hellerman arranged, orchestrated, conducted and sang vaudeville standards, including “When Ragtime Rosie Ragged the Rosary” and “O’Brien Is Tryin’ to Learn to Talk Hawaiian.”

His final composition, his son said, was “Fourths of July,” a patriotic orchestral piece that the New York Jazzharmonic premiered in June.