[1] Born Marie Sundborg in Karlstad, Sweden, Sundelius moved to the United States at the age of nine, ultimately settling in Boston with her family in 1894.

She also had coaching lessons with Swedish composer, Wilhelm Peterson-Berger in Stockholm, and French lyric tenor Edmond Clément in Paris.

The performance was attended by Giulio Gatti-Casazza, the General Manager of the Metropolitan Opera, who immediately approached her afterwards with an offer to join the roster of singers at the Met.

She accepted, and made her opera debut at the "Old Met" on November 25, 1916 as the First Priestess in Christoph Willibald Gluck's Iphigénie en Tauride with Melanie Kurt in the title role and Artur Bodanzky conducting.

Her roles at the house included Anna in Loreley, the Artichoke Vendor in Louise, the Cat in L'oiseau bleu, the Celestial Voice in Don Carlo, Diane in Iphigénie en Tauride, Elvira in L'italiana in Algeri, a Flower Maiden in Parsifal, Gerhilde, Gutrune, and Helmwige in The Ring Cycle, Inès in L'Africaine, Jemmy in William Tell, the Mermaid in Oberon, Micaela in Carmen, Musetta in La bohème, Nedda in Pagliaci, Pomone in La reine Fiammette, Samaritana in Riccardo Zandonai's Francesca da Rimini, Sophie in Der Rosenkavalier, the Voice of a Priestess in Aida, and the title role in The Golden Cockerel.

She also created roles in several world premieres at the Met, including Johanna in Reginald De Koven's The Canterbury Pilgrims (1917), Amy Everton in Charles Wakefield Cadman's Shanewis (1918), the Monitress in Suor Angelica (1918), and Ciesca in Gianni Schicchi (1918).