Cassilly "was a mainstay in the heldentenor repertory in opera houses around the world for 30 years", and particularly excelled in Wagnerian roles like Tristan, Siegmund and Tannhäuser, and in dramatic parts that required both stamina and vocal weight, such as Giuseppe Verdi's Otello and Camille Saint-Saëns's Samson.
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.
In 1965 he launched a major international opera career when he portrayed the title role in a critically acclaimed production of Heinrich Sutermeister's Raskolnikoff at the Grand Théâtre de Genève.
[3] Born in Washington, D.C., Cassilly spent his childhood on a farm near Aberdeen, Maryland where he attended Bel Air High School where his voice potential was first recognized.
Cassilly's performance in The Saint of Bleecker Street drew the attention of Joseph Rosenstock, director of the New York City Opera (NYCO), who invited him to audition for the company in the Spring of 1955.
He notably sang in several American premieres with the company including Ferdinand in Frank Martin's The Tempest in 1957 and the Jailer in Luigi Dallapiccola's Il prigioniero, with Norman Treigle and Leopold Stokowski, in 1960.
His other roles with the NYCO included Don José in Carmen, Edgar Linton in Carlisle Floyd's Wuthering Heights, Paco in La vida breve, Peter Quint in The Turn of the Screw, Pinkerton in Madama Butterfly, Sam Polk in Susannah, the Soldier in Bucci's Tale for a Deaf Ear, the Tenor in Hugo Weisgall's The Tenor, and the title roles in Stravinsky's Oedipus rex among others.
His roles with the AOS included Percy in Anna Bolena (1957), Tebaldo in I Capuleti e i Montecchi (1958), Énée in Les Troyens (1960), Orombello in Beatrice di Tenda (1961), and Pylade in Iphigénie en Tauride (1965).
This was followed by his European debut in July 1958 portraying the role of Sam Polk in Carlisle Floyd's Susannah at the Brussels World's Fair in a production transported from the NYCO.
The following October he made his debut with Montreal Opera singing Radames with conductor Zubin Mehta, Virginia Zeani is Aida and Lili Chookasian as Amneris.
In April 1967 he made his Seattle Opera debut singing Manrico to Eileen Farrell's Leonora and Sherrill Milnes's Count DiLuna in Verdi's Il Trovatore.
That summer he performed at Lincoln Center in Washinginton D.C. with the Hamburg State Opera's transport productions of Mathis der Maler (as the Archbishop) and Jenůfa.
In October 1967 he returned to Vancouver Opera to sing the role of Dick Johnson for the first time in Puccini's La fanciulla del West with Dorothy Kirsten as Minnie, Chester Ludgin as Jack Rance, and Fausto Cleva conducting.
On February 16, 1968, Cassilly made his debut at the Royal Opera, Covent Garden as Laca Klemeň to Marie Collier's Jenůfa with Astrid Varnay as the Kostelnicka.
In 1972 Cassily returned to the San Francisco Opera to sing Radames in Aida and the mayor in the American premiere of Gottfried von Einem's The Visit of the Old Lady.
On January 20, 1973, he made his highly anticipated Metropolitan Opera debut as Radamès opposite Lucine Amara as Aida, Irene Dalis as Amneris, Cornell MacNeil as Amonasro, and Giorgio Tozzi as Ramfis.
The following summer he sang the role of Jason in Luigi Cherubini's rarely heard Médée at the Caramoor International Music Festival, which was also transported to New York City for a performance as of part the NYCO opera season.
In 1975 Cassily sang the Siegmund opposite Birgit Nilsson's Bruennhilde at the Orange Festival, a pairing which was repeated the following year at the Palau de la Música Catalana in Barcelona.
His returned with a critically acclaimed performance of the title role in Wagner's Tannhäuser on January 26, 1978, with Teresa Kubiak as Elisabeth, Bernd Weikl as Wolfram, and Grace Bumbry as Venus.
Among the many roles he portrayed at the house during this time are Aegisth (1980–1984), Canio (1980–1981), Captain Vere in Billy Budd (1980–1989), Don José (1980), Drum Major (1980–1989), Herod (1981–1990), Jimmy Mahoney in Rise and Fall of the City of Mahagonny (1979–1984), the title role in Œdipus rex (in John Dexter's production, 1981), Otello (1978–1980), Peter Grimes (1983), Samson (1981, opposite Viorica Cortez), Tannhäuser (1978–1987), and Tristan in Tristan und Isolde (1981).
He returned to the Palais Garnier in July 1982 to sing Cannio followed by a portrayal of the mystical shepherd in Karol Szymanowski's King Roger with Wolf Trap Opera in August.
In March 1983 Cassilly sang the role of Luka Kuzmič for the American premiere of Leoš Janáček's From the House of the Dead in a concert with the New York Philharmonic.