Sills was a recitalist, especially in the final decade of her career. She sang in mid-size cities and on college concert series, bringing her art to many who might never see her on stage in a fully staged opera. She also sang concerts with a number of symphony orchestras. Sills continued to perform for New York City Opera, her home opera house, essaying new roles right up to her retirement, including the leading roles in Rossini's "Il Turco in Italia", Franz Lehár's "The Merry Widow" and Gian Carlo Menotti's "La Loca", an opera commissioned in honor of her 50th birthday. "La Loca" was the first work written expressly as a vehicle for Sills and was her last new role, as she retired the following year. Her farewell performance was at San Diego Opera in 1980, where she shared the stage with Joan Sutherland in a production of "Die Fledermaus".