Standing at 6'3" and possessing a 250-pound frame The New York Times described him as "a burly tenor with a bright ping on the top notes who had a supple lyric quality [to his voice]", and "was known to bring a musical intelligence and uncommonly clear diction to his work."Cassilly spent the early years of his opera career singing primarily with the New York City Opera between 1955–1966, often portraying roles in obscure and contemporary operas.
During these years he also traveled frequently throughout North America, appearing with most of the major opera companies in the United States and Canada.
In 1978 he joined the roster of principal tenors at the Metropolitan Opera in New York City, where he spent the majority of his time until his retirement in 1990.