In 1978, Sills announced she would retire on October 27, 1980, in a farewell gala at the New York City Opera. In the spring of 1979, she began acting as co-director of NYCO, and became its sole general director as of the fall season of that year, a post she held until 1989, although she remained on the NYCO board until 1991. During her time as general director, Sills helped turn what was then a financially struggling opera company into a viable enterprise. She also devoted herself to various arts causes and such charities as March of Dimes and was sought after for speaking engagements on college campuses and for fund raisers.