High bass Mario Petri enjoyed considerable success in his native Italy during the 1950s and 1960s, essaying a variety of bass roles ranging from those of bel canto composers to early and mid-period works of Verdi. His not unattractive timbre was somewhat wooly in texture and he lacked the sheer vocal clout to compete with such low-voiced luminaries as Boris Christoff, Nicola Rossi-Lemeni, and Giulio Neri, then prominent on Italian stages. Nonetheless, his lithe and muscular appearance made him well-suited to a role such as Don Giovanni and his appearances as the Don won some positive notices. On-stage and on disc, his undoubted apogee occurred with a production and subsequent recording of Rossini's L'Italiana in Algeri with mezzo soprano Giulietta Simionato and conductor Carlo Maria Giulini. In the latter portion of his career, Petri attempted some true baritone roles, but without the security he had shown in the bass register.
By 1950, Petri had established himself as a leading bass. He was engaged by the Glyndebourne Festival for the 1951 season; singing eight performances of the title role in Don Giovanni at the Sussex theater before traveling with the company for nine more appearances in the production at the Edinburgh Festival. In 1953, he appeared as Don Giovanni at La Scala and, in that same season, sang Creonte in Medea with Maria Callas at Florence. In February 1953, however, conductor Herbert von Karajan experienced difficulties in coaching the bass in the quasi-recitative passages of Michael Tippett's A Child of Our Time. Not even the composer himself was able to make Petri comfortable with the score.
In 1955, Petri sang in a Maggio Musicale production of Tancredi received poorly by the critics; Petri's performance in Rossini's Pietra del Paragone, however, was much admired. During the 1950s, Petri was variously a member of several prominent Italian companies; in addition to La Scala and Florence, he sang in Rome, Catania, and at the Regio Emilia.
Most of Petri's recordings were taken from live performances, both on-stage and through radio broadcasts. A 1951 I Lombardi from Italian Radio, a 1958 Khovanshchina with Christoff and Rodzinski, a 1957 Nerone from Naples, and a 1968 Semiramide with Sutherland are all representative of Petri's art.