In 1913, Melchior made his debut in the baritone role of Silvio in Ruggero Leoncavallo's Pagliacci at the Royal Theatre (Det Kongelige Teater) in Copenhagen.
One night, while on tour, Melchior helped an ailing soprano performing in Il trovatore by singing a high C in the Act IV Leonora-di Luna duet.
The Azucena of that performance, the American contralto Mme Charles Cahier, was impressed by the tone she had heard and gave her young colleague sound advice: he was no baritone, but a tenor "with the lid on."
In 1920, Melchior visited England to sing in an experimental radio broadcast to the Scandinavian capital cities from the Marconi station in Chelmsford.
In September 1920, when he was singing the Steersman's Song, from Wagner's Der fliegende Holländer at a Prom Concert, he met the popular novelist and passionate Wagnerite Hugh Walpole and the two quickly became firm friends, travelling together and staying in each other's houses.
Walpole provided the fledgling Heldentenor with financial aid in February 1922, paying in advance two-thirds of the fees for his studies under Victor Beigel.
In 1923, Walpole gave Melchior a further £800, enabling him to continue his studies with Ernst Grenzebach and the legendary dramatic soprano of the Vienna Court Opera, Anna Bahr von Mildenburg.
He sang Tannhäuser opposite Maria Jeritza, Friedrich Schorr, Karin Branzell, and Michael Bohnen with Artur Bodanzky conducting.
To build up his repertory and gain more stage experience, he accepted an engagement at the Hamburg State Opera, where he appeared as Lohengrin, Otello, Radames in Aida and Jean van Leyden in Le prophète.
Some of Melchior's most notable colleagues in the opera houses of the world included the sopranos Frida Leider, Kirsten Flagstad, Lotte Lehmann, Helen Traubel, Marjorie Lawrence, and Elisabeth Rethberg, and conductors Felix Weingartner, Bruno Walter, Wilhelm Furtwängler, Fritz Reiner, Sir Thomas Beecham, Arturo Toscanini, Erich Leinsdorf, George Szell, and Otto Klemperer.
[3] Between 1944 and 1952, Melchior performed in five Hollywood musical films for Metro-Goldwyn-Mayer and Paramount Pictures and made numerous US radio and television appearances.
In 1952, Melchior performed at New York's Palace Theatre, following the popular singer-actress Judy Garland, after she set a record-breaking vaudeville engagement there which lasted nineteen weeks.
In the summer of 1972, Melchior conducted the San Francisco Opera Orchestra at Sigmund Stern Grove in the Radetzky March by Johann Strauss I as part of the 50th anniversary celebration of the company.