He began taking private voice lessons with Harry McGee Cooper after joining his high school's glee club during his senior year.
In 1933, while studying dentistry at Creighton University, Cassel was provided with the opportunity to sing for renowned baritone Lawrence Tibbett after attending one of Tibbet's recitals in Omaha, Nebraska.
Strongly influenced by this encounter, Cassel headed for New York City with just "$40 in his shoe, a pair of coveralls and a briefcase full of music"[1] and traveling by hitching a ride on a cattle train.
He had almost made it back to Broadway Best two years earlier when he created the role of Edvard Grieg in Song of Norway for its pre-Broadway run at the Los Angeles Civic Light Opera.
[6] He made his debut with the Philadelphia La Scala Opera Company as Escamillo in Georges Bizet's Carmen on October 5, 1946 with Bruna Castagna in the title role.
[7] Shortly after his Town Hall recital, Cassel was approached by Laszlo Halasz to join the roster of principal singers at the New York City Opera (NYCO).
Cassel jumped at the chance and made his debut with the company on April 1, 1948 as Scarpia in Giacomo Puccini's Tosca opposite Wilma Spence in the title role and Rudolph Petrak as Cavaradossi.
In 1955 Cassel left the NYCO to rejoin the roster at the Met by the invitation of Rudolf Bing, making his first appearance under his new contract at the house as Scarpia to the Tosca of Renata Tebaldi and Cavaradossi of Giuseppe Campora on March 8, 1955.
His final and 275th performance at the Met was as Kurwenal in Richard Wagner's Tristan und Isolde with Jess Thomas and Birgit Nilsson in the title roles.
[5] In 1958, he appeared as a contestant on the TV game show To Tell the Truth shortly after he had sung three roles at the Met -- Baron Scarpia in Tosca, Kurwenal in Tristan und Isolde and John the Baptist in Salome -- within 24 hours.