Elliot was born Ellen Naomi Cohen to Philip and Bess Cohen in Baltimore, Maryland. She had a younger sister, Leah, who also became a singer as a member of the Coyote Sisters.

The Cohens later moved to Alexandria, Virginia, a suburb of Washington, D.C.. She adopted the name "Cass" in high school—possibly, as Denny Doherty tells it, borrowing it from the actress Peggy Cass, but in any case, it was "Cass", not "Cassandra." She assumed the surname Elliot sometime later, in memory of a friend who had died. While still attending George Washington High School, she became interested in acting and was cast in a school production of the play The Boy Friend. She left high school shortly before graduation and relocated to New York City to further her acting career. She toured in the musical The Music Man, but lost the part of Miss Marmelstein in I Can Get It for You Wholesale, to Barbra Streisand in 1962.

Elliot, known for her sense of humor and optimism, was considered by some to be the most charismatic member of the group. Her powerful, distinctive voice was a large factor in their success. She is best remembered for her vocals on the group's hits "California Dreamin'," "Monday Monday," and "Words of Love," and particularly for the solo "Dream a Little Dream of Me," which the group recorded in 1968 after learning about the death of Fabian Andre...