Tajo was born in Pinerolo, Piedmont, and studied violin and voice at the Music Conservatory of Turin with Nilde Stichi-Bertozzi. He made his stage debut in 1935, as Fafner (Das Rheingold), under Fritz Busch. At Busch's invitation, he followed him to Glyndebourne, where he became a member of the chorus, also appearing in comprimario roles.
In 1939, he was back in Italy, where he became a member of the Rome Opera, in 1942 taking part in the Italian premiere of Berg's Wozzeck. In 1940, he joined the Teatro alla Scala in Milan, where he sang regularly until 1956. He appeared with the Maggio Musicale Fiorentino in 1942, as Leporello in Don Giovanni, a role he would sing numerous times during his career. In 1961, the bass was in the world premiere of Luigi Nono's Intolleranza 1960, in Venice.
The war over, his career quickly took an international turn, with debuts in Paris, London, Edinburgh and Buenos Aires. In 1946, he made his debut at the Lyric Opera of Chicago, and 1948 saw his debut at both the San Francisco Opera and Metropolitan Opera in New York (Don Basilio in The Barber of Seville, with Giuseppe Valdengo); his roles there included Figaro, Leporello, Don Basilio, Dulcamara, Don Pasquale, Gianni Schicchi, etc.
Although he made a specialty of comic roles, he sang a fair number of serious roles, notably Verdi's Attila and Banco, and Mussorgsky's Boris Godunov. He also created Samuel in Darius Milhaud's David, as well as roles in operas by Berio, Lualdi, Malipiero and Nono. In 1953 he appeared at the Teatro Comunale Florence as Count Rostov and Field-Marshal Kutuzov in the near-complete Italian-language première of Prokofiev's War and Peace.
In 1966, he began teaching at the University of Cincinnati, where he was largely responsible for the establishment of an opera workshop. He continued singing until well into his seventies, mostly in character roles such as Geronte, Benoit, Alcindoro, and the Sacristan. His last stage appearance was in 1991.
Tajo made relatively few recordings, the most famous being the 1950 RCA Victor Rigoletto, with Leonard Warren, Erna Berger and Jan Peerce, conducted by Renato Cellini. He also recorded The Marriage of Figaro and Don Giovanni for Cetra. He can also be heard in live broadcasts of Macbeth, opposite Maria Callas, conducted by Victor de Sabata; and the Florence War and Peace.
In the late 1940s, he appeared in film versions of The Barber of Seville, L'elisir d'amore, and Lucia di Lammermoor, as well as in television production such as Don Pasquale in 1955, released as a Hardy Classic Video (DVD).
In 1989, he appeared in Francesca Zambello's production of La bohème, as Benoit and Alcindoro, which was a film of a San Francisco Opera production starring Mirella Freni and Luciano Pavarotti. It was released on Kultur video.
As a special guest, he sang the role of Don Pasquale during the Metropolitan Opera Centennial Celebration in October 1983 with his former student, the American opera Soprano Barbara Daniels, to great delight of his protege and the audience in attendance.
- Le guide de l'opéra, les indispensables de la musique, R. Mancini & J-J. Rouvereux, (Fayard, 1986), ISBN 2-213-01563-5