Hasslo studied in Stockholm with Hjaldis Ingebjart and Joseph Hislop and made his debut at the Stockholm Opera, as Guglielmo in Cosi fan tutte, in 1940, where he remained until 1964, quickly establishing himself as first baritone.
He was particularly admired in the Italian repertory singing roles such as Figaro in Il barbiere di Siviglia, Belcore in L'elisir d'amore, Malatesta in Don Pasquale, the title role in Macbeth and Rigoletto, di Luna in Il trovatore, Riccardo in Un ballo in maschera, Marcello in La boheme, etc. He also sang in Mozart and Wagner operas, and attempted the tenor role of Cavaradossi in Tosca, in 1943.
Hasslo was also much appreciated in concert and often appeared in oratorios.
- 1952 - Tchaikowsky - Eugen Onegin - Hugo Hasslo, Rudolf Schock, Sena Jurinac, Gisela Litz, Gottlob Frick - Hamburg Radio Chorus and Orchestra, Wilhelm Schüchter - Cantus Classics (sung in German)
- 1953 - Verdi - Don Carlo - Aase Nordmo-Lövberg, Dagmar Herrmann, Libero de Luca, Hugo Hasslo, Josef Greindl, Hans-Herbert Fiedler - Hamburg Radio Chorus and Orchestra, Wilhelm Schüchter - Cantus Classics (sung in German)
- 1959 - Verdi - Rigoletto - Hugo Hasslo, Margareta Hallin, Nicolai Gedda, Kerstin Meyer, Arne Tyren - Stockholm Opera Chorus and Orchestra, Sixten Ehrling - (live recording issued on various labels)