In 1947, she made her operatic stage debut as the Spanish gypsy Frasquita in Bizet's "Carmen" with the Philadelphia Civic Grand Opera Company. She toured North America with the Charles Wagner Opera Company, in the fall of 1951 singing Violetta in "La traviata" and, in the fall of 1952, singing Micaëla in "Carmen". On September 15, 1953, she made her debut with the San Francisco Opera as Helen of Troy in Boito's "Mefistofele" and also sang Donna Elvira in "Don Giovanni" the same season. In a step outside of the repertoire she is commonly associated with, Sills gave four performances of the title role of "Aida" in July 1954 in Salt Lake City. On October 29, 1955, she first appeared with the New York City Opera as Rosalinde in Johann Strauss II's "Die Fledermaus", which received critical praise. As early as 1956 she performed before an audience of over 13,000 at the Lewisohn Stadium with the noted operatic conductor Alfredo Antonini in an aria from Bellini's "I puritani". Her reputation expanded with her performance of the title role in the New York premiere of Douglas Moore's "The Ballad of Baby Doe" in 1958.