Carol Theresa Vaness was born on July 27, 1952, in Los Angeles, California. When she was a teenager, her music-loving father insisted she take piano lessons. She continued studying piano when she entered California State Polytechnic College, but also began taking an interest in the voice. She began taking voice lessons in her late-teens, and later studied in graduate school with David Scott, head of California State University's voice and opera department.
Although Vaness was convinced that she was a mezzo-soprano, Scott trained her as a soprano, and she learned the role of Tosca by the end of their first year together. In 1976, she became one of six singers chosen to take part of the inagural year of the San Francisco Opera's Merola Opera Program for young singers. Her apprenticeship lasted two years, during which she studied with Kurt Adler, then the general director of the San Francisco Opera. Adler advised Vaness to resist offers to sing major parts at such an early stage in her career, and as a result she turned down offers from the opera companies in Houston; Tuson, Arizona; and Glyndebourne.
In 1977, Vaness was singing the small part of Henrietta in Bellini's I Puritani with Beverly Sills as Elvira. Sills was impressed by Vaness' voice, and arranged an audition for her with the conductor Julius Rudel, who engaged Vaness to sing Vitellia in La Clemenza di Tito with the New York City Opera in October 1979. She returned to the New York City Opera as Donna Anna in Don Giovanni and as Mistress Ford in The Merry Wives of Windsor. Although neither of these productions were a great success, Carol Vaness scored a major triumph in October 1980 as Antonia in Offenbach's The Tales of Hoffmann.
In the winter of 1980, Carol Vaness made her European debut as Vitellia in Bordeaux, France, and then returned to New York to sing Vitellia and Mistress Ford, as well as Mimi in La Bohème and Leila in Bizet's Les Pêcheurs de Perles at the City Opera. Soon after, in November 1981, she sang her first Gilda in Rigoletto, and in January 1982 undertook her first Countess in Philadelphia Opera's production of Mozart's Le Nozze di Figaro. She made her Glyndebourne Festival debut in the summer of 1982 as Donna Anna and her Covent Garden debut shortly after as Mimi in La Bohème.
In February 1984, Carol Vaness made her Metropolitan Opera debut as Armida in Handel's Rinaldo. That July, she returned to Glyndebourne as Fiordiligi in Cosi Fan Tutte, a role she repeated later that year at the Met. She returned to the Met in the 1985-86 season as the Countess in Le Nozze di Figaro and as Elettra in Idomeneo. In recent years, she has added new roles to her repertoire, including the title role in Norma, Elizabeth de Valois in Don Carlos, and Desdemona in Verdi's Otello.