He was born in a poor family Born in the Upper Carniolan village of Kropa, in what was then the Austro-Hungarian Empire (now in Slovenia). He went to the conservatory in Ljubljana with the intention of studying composition and organ, but in 1934 he received a scholarship which sent him to Vienna. There, he devoted himself exclusively to vocal study with Marie Radó.
Dermota made his debut at the opera in Cluj in 1934, and was promptly invited by Bruno Walter to perform at the Wiener Staatsoper. Here he made his début as a Man in Armor in Wolfgang Amadeus Mozart's The Magic Flute in 1936 and got a contract immediately. His first leading role was Alfredo in Giuseppe Verdi's La Traviata, which he sang in 1937. In the same year Dermota made his début at the Salzburg Festival in a production of Richard Wagner's "Die Meistersinger von Nürnberg", conducted by Arturo Toscanini.
Dermota quickly became a favorite of the viennese audience and would remain with the State Opera's company for more than forty years. He was a witness (and helped to save parts of the furniture) when the opera house burned down after an allied air raid on March 13, 1945, he stayed with the company in its provisional lodgings at Theater an der Wien, and was one of the stars of the reopening of the original house in 1955 (as Florestan in Ludwig van Beethoven's Fidelio). As early as 1946 Dermota was honoured for his faith with the title of Kammersänger.
For 20 years Dermota sang at the Salzburg Festival almost every summer. As guest he gave acclaimed performances at Royal Opera House Covent Garden London, in Paris and Rome, at Teatro di San Carlo in Naples, Teatro Colón in Buenos Aires, in Australia, Czechoslovakia and Hungary.
Dermota was best known for his Mozart roles - especially his Don Ottavio in Don Giovanni. But he sang a good deal of the lyric tenor repertory during his career, including more modern parts, such as Oedipus in Igor Stravinsky's Oedipus Rex, the title role of Hans Pfitzner's Palestrina and Flamand in Richard Strauss's Capriccio. Later in life he ventured into heldentenor territory, essaying parts such as the title role in Bedřich Smetana's Dalibor and Florestan. All told, his repertoire included some 80 roles.
To celebrate his 70th birthday, Dermota sang Tamino in The Magic Flute at the Staatsoper. A year later he sang the Shepherd in Carlos Kleiber's famous recording of Richard Wagner's Tristan und Isolde, sounding astonishingly young.
He died in Vienna less than a month after his 79th birthday.