He arranged music for many singers and groups, including Harry Belafonte and Peter, Paul and Mary, and worked with Milt Okun.
His trumpet playing ended during World War II, when a German mortar shell nearly severed his right wrist while his Army infantry unit was advancing toward the Rhine River.
While recovering at a hospital on Staten Island, he began singing with the CIO chorus, which was where he met and started a lifelong association with Pete Seeger.
Because of McCarthyism, and the fact that he had joined the Communist Party as a young man, De Cormier used the name Robert Corman as a pseudonym on many Harry Belafonte recordings.
He and his wife, actress and singer Louise De Cormier, collected and recorded folk songs from the Catskill Mountains of New York.
De Cormier taught a class at Saint Michael's College in Colchester, Vermont, entitled "Songs of Resistance: Music in Struggle" in 2008.