During her 35-year career as an opera singer, the Brooklyn-born Sills performed in the world's leading houses, including La Scala, Covent Garden, the Metropolitan Opera, and New York City Opera. She starred in such operas as Guilio Cesare, Manon, Lucia di Lammermoor, La Traviata, and The Siege of Corinth.
Sills became director of City Opera in 1979, where she expanded the repertory and pioneered the use of supertitles during her 10-year tenure. She later became chairwoman of the board of Lincoln Center and chairwoman of the Metropolitan Opera.
She was a frequent presence on television, appearing with her good friend, Carol Burnett, in a television special, as well as making appearances on programs as diverse as The Tonight Show and The Muppet Show. She also hosted many Live from Lincoln Center broadcasts.
Sills' husband, Peter Greenough, passed away last year. She is survived by their two children, Meredith and Peter, and three stepchildren.