He went on to sing in the world premieres of Ward's The Crucible (starring Chester Ludgin), Floyd's The Passion of Jonathan Wade (with Phyllis Curtin), and Menotti's The Most Important Man (conducted by Christopher Keene).
By 1970, Theyard was one of the City Opera's leading tenors, appearing in Madama Butterfly, Mefistofele (opposite Norman Treigle), Louise, The Makropoulos Case, Carmen, Susannah, Tosca (with Galvany), Cavalleria rusticana, Les contes d'Hoffmann, Turandot and La fanciulla del West.
In 1974, he debuted with the Metropolitan Opera for an impressive string of operas: Les contes d'Hoffmann (with Dame Joan Sutherland), Manon Lescaut, Madama Butterfly, Cavalleria rusticana, Grigori in Boris Godunov, La forza del destino (directed by John Dexter), L'assedio di Corinto (Beverly Sills's house debut) and Il tabarro (with Renata Scotto).
From 1976, he returned to his native city for starring roles in Manon Lescaut, Lucia di Lammermoor, Andrea Chénier, La bohème and Carmen (opposite Shirley Verrett).
Mr Theyard also appeared on Broadway, in the original productions of Man of La Mancha (as Anselmo the muleteer, 1965) and A Time for Singing (as Owen Morgan, 1966).