In 1922, Louis Gruenberg suggested that Claire Reis become the executive secretary of the International Composers' Guild.

[2] After hosting the American premiere of Arnold Schoenberg's Pierrot Lunaire on 4 February 1923, she proposed staging a repeat performance, contrary to a rule of the ICG emanating from Edgard Varese that aside from an immediate encore, no musical piece should be scheduled by the ICG twice.

Reis was a prolific author of many articles on music, two catalogs for the International Society for Contemporary Music, and the books Composers, Conductors and Critics in 1955 and Composers in America: Biographical Sketches in 1938; the latter is a valuable reference work which appeared in a second, enlarged edition in 1947 and was republished in 1977.

Reis served on the board of directors of the New York City Center of Music and Drama.

She also helped to found the Women's City Club and was a member of the board of the Work Projects Administration.