Theyard is a 1957 graduate of Loyola University of the South, where he studied under Dorothy Hulse, who was also the teacher of Audrey Schuh and Charles Anthony. He began his career with the New Orleans Opera Association in secondary parts, including appearances in "Salome" (1949), "Le nozze di Figaro" (1956), "Il trovatore" (opposite Herva Nelli, 1958) and "The Beggar's Opera" (directed by Lillian Gish, 1958). Later, in New York, he studied under Armen Boyajian, also the pedagogue of Marisa Galvany, Paul Plishka, and Samuel Ramey.