She retired from performing at the age of 50, with an appearance in Menotti's La Loca, and accepted the position of General Manager of the New York City Opera. In 1991, she joined the board of the Metropolitan Opera, and four years later became head of New York's Lincoln Center. Sills sang regularly in concerts and recitals containing the arias from her famous roles. Her concert performance of the first version of Richard Strauss' Ariadne auf Naxos is justly famous, since Zerbinetta's aria in this version is much more difficult than in the revised version. Her basic voice was a light, high soprano with excellent technique and breath control. She was best heard in roles where fragility of character was paramount, such as Marie in Donizetti's La Fille du régiment, Puccini's Manon Lescaut, and Violetta. By sheer power of character she held her own in operas normally best served by larger voices as well.
Although her artistic life was filled with great triumphs, Sills knew personal tragedy. Her daughter was born deaf and her son is mentally retarded. She was active in the March of Dimes Mothers' March on Birth Defects and other related organizations. Her autobiography was published in 1976 with the title Bubbles: A Self-Portrait and was revised in 1981 as Bubbles: An Encore; another autobiography, Beverly, followed in 1987.