He composed his first theatre work, a little opera buffa Il Dottor Bartolo Salsapariglia, at the age of 12, in which he wished to take part in the basso comico role, though he had then only an alto voice.
His own one-act opera Il biricchino (Venice 1892) did not fare well in Vienna, for after it was presented at the Teatro dell’Esposizione in 1892, the critic Hanslick pronounced it to be a mediocre and objectionable thing.
Mugnone became famous for other revivals of important operas, including Gluck's Orfeo, Rossini's Guglielmo Tell, and Berlioz's La Damnation de Faust in its original form as an oratorio.
Other Italian premieres of this period given by him were:[6] It was through Sonzogno that Mugnano began to develop his work in operatic seasons in Argentina and elsewhere in South America, where he made a significant impact.
[7] Between May and August 1919 he conducted a Covent Garden Italian season, including Aida, Tosca, Madame Butterfly and other works, and the first England performance of Mascagni's Iris (July, with Margaret Sheridan).
This appearance was under the management of Thomas Beecham, who describes him as a man of fiery and uncontrollable temper... 'never a day passed without a stormy scene with singers, chorus and orchestra, coupled with threats to return to Italy at once.'
A long speech of justification, explanation, and declaration of fondness for England followed, and the season proceeded: life was a little quieter after that, at least for Beecham, who considered Mugnone's interpretations of Verdi the finest he ever heard.
From May to July 1925 he was back at Covent Garden for Aida, Andrea Chénier (with Margaret Sheridan, Giacomo Lauri-Volpi, Benvenuto Franci), Il barbiere di Siviglia (with Toti dal Monte) and Tosca (with Maria Jeritza).
Around 1933 Mugnone gave to the Museums of La Scala and the Rome Opera, and to the Naples Conservatorio, about 2000 documents including letters of Verdi, Massenet, Mascagni, Strauss and Leoncavallo, and a voluminous cache of Puccini materials.