Nilsson made her name singing grueling Wagnerian roles such as Brunnhilde, starting with her remarkable debut at the Metropolitan Opera in 1959 as Isolde in Wagner's Tristan und Isolde, and her Bayreuth debut as Else in Lohengrin that same year. She was also much-lauded for her performances in Strauss's Salome and Elektra, and Puccini's Turandot.
Nilsson astounded audiences and critics with her enormous vocal power, her remarkable ability to hold high notes for a long time, and her interpretative talent. She was often described as a successor of equally acclaimed Norwegian soprano Kirsten Flagstad, who was a regular pre-World War II Wagnerian at the Met Opera.
Nilsson was born in 1918 in Vastra Karup, near the southern Swedish town of Malmo and studied at the Royal Academy of Music in Stockholm. She made her debut at the Stockholm Royal Opera in 1946 as Agathe in Weber's Der Freisch‹tz; her first important foreign engagement was in 1951 at Glyndebourne, as Elettra in Mozart's Idomeneo.
She was noted for her sense of humor; when preparing her taxes and asked if she had any dependents, she reportedly said "Yes, just one, Rudolf Bing," referring to the longtime director of the Metropolitan Opera.
Nilsson married Swedish restaurateur Bertil Niklasson in 1949; the couple had no children. She retired in the mid-1980s and was an active teacher. She returned to the stage of the Metropolitan Opera for the 1996 gala celebrating conductor James Levine's 25th anniversary with the company.
In a rare statement, King Carl XVI Gustaf said, "With Birgit Nilsson's passing, Sweden has lost one of its greatest artists."
A private funeral was held January 11 at a church in her hometown of Vastra Karup.