Sarah Dash, a founding member of the groundbreaking, million-selling vocal trio Labelle, died on Monday. She was 76.

Her death was announced on social media by Patti LaBelle and Nona Hendryx, the other members of Labelle. They did not say where she died or what the cause was.

Ms. Dash brought her church-rooted soprano and high harmonies to Labelle, which began as a 1960s girl group before reinventing itself as a socially aware, Afro-futuristic rock and funk powerhouse, costumed in glittery sci-fi outfits and singing about revolution as well as earthy romance. In 1974, Labelle had a No. 1 hit, “Lady Marmalade,” and performed the first concert by a pop group — and a Black group — at the Metropolitan Opera House in New York City.

In Ms. LaBelle’s 1996 autobiography, “Don’t Block the Blessings,” she wrote, “It was perfect harmony, the way we sounded together, the way we fit together, the way we moved together.”

Ms. Hendryx, speaking by phone on Monday, described Ms. Dash as “a little ball of energy.” She added that Ms. Dash had played a crucial role in Labelle’s vocal interplay.

“Sarah was very meticulous about vocal parts,” Ms. Hendryx said. “Patti and I would just want to do whatever we wanted to do, and Sarah had really great ears and was really great with harmony. That was her strength. She was the glue.”

Sarah Dash was born in Trenton, N.J., on Aug. 18, 1945, the seventh of 13 children of Abraham and Mary Elizabeth Dash. Her father was a pastor, her mother a nurse. She grew up singing in the Trenton Church of Christ choir and turned to secular music as a teenager. She met Ms. Hendryx when the two girls’ church choirs shared a bill, and invited her to join her in the Del-Capris, a local doo-wop quintet.

In 1961, Ms. Dash and Ms. Hendryx joined Patricia Holte and Cynthia Birdsong, members of a Philadelphia group, the Ordettes, to form a quartet, which they named the Blue Belles. Because there was already another group called the Bluebells, Ms. Holte adopted the name Patti LaBelle and the group became Patti LaBelle and the Bluebelles (sometimes spelled Blue Belles or Bluebells).

Their first hit was not actually by them; “I Sold My Heart to the Junkman” was recorded by a Chicago girl group, the Starlets. But because of contractual complications, the single was credited to the Bluebelles, who performed it on tour and on television.

The Bluebelles had minor hits of their own with gospel-charged versions of standard songs including “You’ll Never Walk Alone” and “Danny Boy,” and the group worked through the 1960s on the R&B circuit, recording on the Newtown, Cameo-Parkway and Atlantic labels. For years, they played three shows a night, up to 300 nights a year, at clubs and theaters; in New York City, they became known as the Sweethearts of the Apollo.

Ms. Birdsong left the group to join the Supremes in 1967, but the trio persevered. In 1966, the group had performed on the BBC pop program “Ready, Steady, Go!,” and the members had stayed in contact with a producer from the show, Vicki Wickham. Ms. Wickham became their manager, along with the Who’s management team, Chris Stamp and Kit Lambert.

The Bluebelles metamorphosed into Labelle in 1970. Abandoning the formal gowns and wigs of a girl group for jeans, tie-dye and Afros, the group moved from the R&B circuit to rock clubs like the Bitter End in Manhattan.

In 1971, Labelle released its self-titled debut album and collaborated with Laura Nyro on her album “Gonna Take a Miracle”; the group also opened for the Who on an arena tour. The trio’s 1972 album, “Moon Shadow,” started with the Who’s “Won’t Get Fooled Again”; its 1973 album, “Pressure Cookin’,” featured a medley of the Thunderclap Newman song “Something in the Air” and Gil Scott-Heron’s “The Revolution Will Not Be Televised.”

Along with its sociopolitical messages, Labelle adopted a new look designed by Larry LeGaspi: “campy space costumes of channel-quilted metallic leather, disclike cowls and boots with stratospherically high stacked heels,” as Guy Trebay wrote in The New York Times. Labelle was at the forefront of glam-rock and Afro-Futurism.

While Ms. LaBelle’s acrobatic voice often dominated Labelle’s arrangements, Ms. Dash was prominent in songs like “(Can I Speak to You Before You Go to) Hollywood.”

Labelle reached its commercial peak with the 1974 album “Nightbirds,” produced by Allen Toussaint with a New Orleans backup band. Although most of its songs were written by Ms. Hendryx, its hit was by Bob Crewe and Kenny Nolan: “Lady Marmalade,” a tale of a memorable New Orleans prostitute, with the refrain “Voulez-vous couchez avec moi ce soir?”

Labelle made two more albums, “Phoenix” and “Chameleon,” before breaking up in 1977, with its members pulling in different musical directions: disco for Ms. Dash and Ms. LaBelle, rock for Ms. Hendryx. They moved into solo careers, and Ms. Dash started hers with a hit in 1978: “Sinner Man,” from her solo album simply titled “Sarah Dash,” the first of four she made in the 1970s and ’80s. “Oo-La-La, Too Soon,” from her 1980 album “Oo-La-La, Sarah Dash,” was turned into a commercial jingle for Sasson jeans.

She also recorded widely as a session singer — with Nile Rodgers, the Marshall Tucker Band, the O’Jays, Keith Richards and the Rolling Stones. She looked back on her career in the 1990s with one-woman shows and an autobiography, “A Dash of Diva.”

Information on survivors was not immediately available.

Ms. Dash stayed in touch with the members of Labelle and appeared on solo albums by Ms. LaBelle and Ms. Hendryx. The trio had a club hit together in 1995 with “Turn It Out,” heard on the soundtrack of the movie “To Wong Foo, Thanks for Everything! Julie Newmar.” In 2008, Labelle reunited for a full album, “Back to Now,” followed by a tour.

Ms. Dash gave her final performance on Saturday night, two days before her death, when she joined Ms. LaBelle during a performance in Atlantic City.

“Sarah Dash was an awesomely talented, beautiful and loving soul who blessed my life and the lives of so many others in more ways than I can say,” Ms. LaBelle posted on social media. “And I could always count on her to have my back!”