Robert de Cormier, classically trained singer and choral composer (Counterpoint)

Robert De Cormier, a classically trained singer and choral composer who helped spur a folk music revival in New York, worked as an arranger for Harry Belafonte and became almost a “fourth member” of the harmonizing vocal outfit Peter, Paul and Mary, died Nov. 7 at a hospital in Rutland, Vt. He was 95.

The cause was complications from a urinary infection, said his wife and sometime collaborator, singer Louise De Cormier.

A dashing baritone, Mr. De Cormier played a leading — if largely invisible — role in the folk revival of the 1950s and 60s, when he arranged politically progressive songs for artists such as Pete Seeger, Cisco Houston and Paul Robeson, sometimes accompanying them with his voice or guitar.

The teetotalling Seeger became a close friend (Louise De Cormier recalled his staying at their home one night after a concert, guzzling milk on the couch), and a former pupil, Mary Travers, became a longtime collaborator, developing a crush on Mr. De Cormier and asking him to join her new vocal group as music director.

Mr. De Cormier was busy at the time, arranging songs for Belafonte and leading the calypso singer’s backing band. When Travers’s group Peter, Paul and Mary reunited in 1978 after an eight-year hiatus, Mr. De Cormier jumped into the studio with them, remaining with the band for 17 years as a conductor, arranger, “recording secretary” and sometime peacemaker.

"He lived the same imperative that Peter, Paul and Mary did," band member Peter Yarrow told the New York Times in 2016, "thinking of this music not just as an art form that had beauty, and that's admirable in and of itself, but also as a vehicle for spreading a certain kind of sensibility."

That sensibility, a steadfast commitment to peace and social justice, remained a fixture of Mr. De Cormier's musical projects for six decades, extending from his folk arrangements to his work as a composer and director of classical choral groups such as the New York Choral Society, Vermont Symphony Orchestra Chorus and the Vermont-based group Counterpoint.

At the New York Choral Society, where Mr. De Cormier was director from 1970 to 1987, he drew on his folk connections to arrange for choral collaborations with Belafonte and Peter, Paul and Mary, and expanded the group to 175 singers, more than 10 times its initial size. He also diversified the chorus’s songbook, supplementing canonical works by Brahms and Handel with modern pieces Mr. De Cormier sometimes composed himself.

His works included the cantata "The Jolly Beggars," a setting of poetry by Robert Burns, and the dance classic "The Rainbow 'Round My Shoulder," a chain-gang song that he arranged with producer Milt Okun, and drew from genres as varied as African American spirituals and the folk music of Israel, Sweden, Japan and Korea.

In part, his musical sensibility was a result of his left-leaning political views. He began a career as a singer after seeing the Congress of Industrial Organizations, a union federation, was organizing a chorus.

"I was really concerned about the troubles of the world," Mr. De Cormier told Vermont Public Radio in 2002, "and was trying to make up my mind if I wanted to be a union organizer or a musician."

Robert Romeo De Cormier Jr. was born near the Long Island town of Farmingdale on Jan. 7, 1922, and raised in Poughkeepsie, N.Y. His father was a French Canadian shop teacher, and his mother was a Swedish-born guitarist who played in a mandolin band.

Bob, as he was called, began playing the trumpet at 7 and continued while attending Colby College in Maine and the University of New Mexico.

His trumpet playing ended during World War II, when a German mortar shell nearly severed his right wrist, striking him in the arm while his Army infantry unit was advancing toward the Rhine River. Thirteen operations saved his hand.

Mr. De Cormier was recovering at a hospital on Staten Island when he began singing with the CIO chorus. The group’s director suggested he study voice at the Juilliard School in New York, and he graduated in 1948 with a bachelor’s degree, receiving a master’s degree one year later.

Supporting himself with a teaching position at the progressive Little Red School House & Elisabeth Irwin High School, he began playing with groups such as the Jewish Young Folk Singers and joined up with Belafonte in 1957.

Mr. De Cormier arranged songs under an assumed name, Robert Corman, in what he later said was an attempt to safeguard Belafonte from criticism that he was collaborating with a left-leaning labor activist. “I told Harry I didn’t want to jeopardize him in any way,” he later told the Times. “I was on many lists.”

The musicians parted ways in the early 1960s, when Belafonte no longer wanted a backing chorus. Mr. De Cormier re-christened his vocal group the De Cormier Singers, and they toured and recorded for several years. Mr. De Cormier also recorded several children’s albums with the former Louise Dobbs, whom he married in 1950.

In addition to his wife of Belmont, Vt., survivors include a daughter, Robin Timko of Proctorsville, Vt.; two grandchildren; and two great-grandchildren.

A son, Christopher De Cormier, died of cancer in 1977; a piece written by Mr. De Cormier in his honor is scheduled to be performed at a 2018 tribute concert for Mr. De Cormier. Titled “Legacy,” the work’s four movements are written in a kind of code, with each key spelling out Christopher De Cormier’s initials: C major, D minor, E minor and C again.