The list of performers the pianist and arranger Mike Renzi worked with is amazingly long. And varied. Mel Tormé and Lena Horne are on it. Peggy Lee and Maureen McGovern are on it. Lady Gaga and Tony Bennett are on it. Big Bird is on it.

Mr. Renzi, who for decades was one of the most sought-after accompanists in the music business, especially in jazz circles, died on Sept. 28 in Newport, R.I. He was 80.

Memorial Funeral Homes of Rhode Island posted news of his death but did not give a cause. Mr. Renzi lived in Middletown, R.I., and in Boca Raton, Fla.

Mr. Renzi began drawing notice in the 1960s with his trio, which played clubs in his native Rhode Island; by the mid-1970s it was also accompanying singers like Sylvia Syms. In 1976 he relocated to New York, and soon he was in demand as an accompanist, playing piano for the actress and singer Claudia McNeil and others in venues of all sorts. In 1979 he had a memorable moment at Carnegie Hall during a tribute to Hoagy Carmichael when Mr. Carmichael, then 79, joined him onstage and the two played a new Carmichael composition, “Piano Pedal Rag.”

In the 1980s Mr. Renzi was seemingly everywhere. He played in various jazz ensembles; wrote the music for “Louis,” a 1981 stage show about the early career of Louis Armstrong; performed with Mr. Tormé at the Manhattan club Marty’s; backed Ms. Lee in her 1983 Broadway show, “Peg”; and accompanied the tap dancer Harold Nicholas in a 1988 show called “Espectaculo Flamenco Tap-Dance.”

When Mr. Renzi performed he was rarely in the spotlight, but savvy listeners understood that his playing and arrangements helped the stars he was backing be at their best. In 1982, when Mr. Tormé was in the midst of a monthlong residency at Marty’s, Rex Reed wrote in The Daily News that Mr. Renzi was “undoubtedly the most talented and creative piano accompanist in New York.”

“Renzi creates so many tone poems of color behind Tormé’s voice that there’s a floral explosion of music onstage,” he wrote. “Deep dark purples behind ‘Sophisticated Ladies,’ pale blue pastels behind ‘Pieces of Dreams,’ and there were times when I thought I heard harps and woodwinds coming out of the piano.”

That same year William Glackin, reviewing Mr. Tormé’s performance at the Concord Jazz Festival in California for The Sacramento Bee, said that the singer “was matched in intensity and power and sensitivity and inspiration by the extraordinary Renzi, who is one of the greatest jazz pianists I have ever heard.”

For part of his performing career, Mr. Renzi also had a day job: music director for “Sesame Street.” In 2002 he shared a Daytime Emmy Award for outstanding achievement in music direction and composition for that work, the first of four he’d receive for the show.

“It’s a lot of work, but the rewards are great,” he told The Hollywood Reporter in 2006. “When you hear something you wrote sung by a talented Muppeteer and then see how much kids enjoy it, it’s all worth it.”

Michael Ernest Renzi was born on April 28, 1941, in Providence, R.I., to Ernest and Elvia Renzi. He began studying piano at 8. In the early 1960s, after playing in an uncle’s band, he joined the Artones, a band led by the Rhode Island saxophonist Art Pelosi.

In 1962 he formed his own group, which became a fixture at a Providence nightclub, the Kings and Queens.

He received a bachelor’s degree in piano and music education at the Boston Conservatory and studied at the Berklee College of Music. He was working as music director on an afternoon talk show in Boston when he met Ms. Syms, who encouraged him to try New York.

Once there, he landed a job playing at Carnegie Tavern, adjacent to Carnegie Hall, and Mr. Tormé soon recruited him for his appearances at Marty’s; he played on Mr. Tormé’s album “Live at Marty’s” (1981) and a follow-up released the next year. He also played on, or contributed arrangements to, scores of other records, by Ms. Horne, Blossom Dearie, Cleo Laine, Ms. Lee, Ms. Syms, K.T. Sullivan, Brian Stokes Mitchell and others.

He worked frequently with Ms. McGovern. Mr. Tormé had introduced the two in 1981, and she made her first record with Mr. Renzi in the mid-1980s, a voice-and-piano album called “Another Woman in Love.”

At the time Ms. McGovern was best known for “The Morning After,” the Oscar-winning hit from the 1972 movie “The Poseidon Adventure,” but she had since been frustrated by a producer who seemed bent on overproducing her material.

Mr. Renzi, she said, rescued her.

“It wasn’t until I did my first solo and voice album with Mike in 1986 that people said, ‘My God, she really can sing,’” she told The Sunday News of Lancaster, Pa., in 1999.

Their 1998 collaboration, “The Pleasure of His Company,” was nominated for a Grammy.

Mr. Renzi also played piano and was among the music directors on “Cheek to Cheek,” the 2014 collaboration between Lady Gaga and Mr. Bennett, which won a Grammy for best traditional pop vocal album. In addition to his “Sesame Street” Emmys, Mr. Renzi shared three Daytime Emmys for his music direction on the soap opera “All My Children.”

Information on Mr. Renzi’s survivors was not available.

Last week on NPR’s “Weekend Edition,” the broadcaster Murray Horwitz, who knew Mr. Renzi for more than 40 years, recalled working with him on various projects, including a few years ago on the annual Sarah Vaughan International Jazz Vocal Competition in New Jersey. The young singers who competed there, he said, were not used to accompanists with Mr. Renzi’s skill.

“You could just see it in their whole bearing,” he said. “It was like they’d been sleeping on park benches, and now they were in a feather bed at the Ritz.”