Taddei had his first experience of opera at the age of four or five when his mother took him to see Verdi's Otello. The boy soon began entertaining his father's friends with popular Italian songs of the day, and he was assigned solos by his elementary school teacher. When Taddei was eight or nine, he sang near the steps of a church to raise enough money to buy school books for seven of his poor schoolmates. At age 19, Taddei won a vocal competition sponsored by the Rome Opera and shortly thereafter made his debut at that theater. The role was the Herald in Lohengrin, sung under the tutelage of conductor Tullio Serafin. Taddei acknowledged throughout his career the lessons imparted during coaching sessions at Serafin's home with major singers of the era. Several other emerging stars were soon to become good friends, including Italo Tajo and Tito Gobbi.
With Italy enmeshed in WWII, Taddei was conscripted in 1942 and sent to Yugoslavia. In a strange turn of events, he was captured by German troops and taken to a concentration camp where his fate remained a mystery for several months. Once released, Taddei found favor among the American troops and officers for whom he sang often, and their support greatly assisted the baritone in restarting his career. For example, a concert in Vienna led to a three-year contract there for the two Figaros, Amonasro, and Rigoletto. To entreaties that he come to America, however, Taddei responded that he wished to conquer Italy first, especially La Scala.
Other important engagements followed quickly. Taddei's London debut took place at the Cambridge Theatre (with Jay Pomeroy's Anglo-Italian company) as Rigoletto and Scarpia. His Salzburg debut in 1948 was as Mozart's Figaro and that same year, he sang at La Scala, beginning an association that lasted until 1961. In Italy, the singer performed extensively in the Wagnerian repertory, particularly the role of Hans Sachs. A series of Cetra recordings brought Taddei an American following long before he sang there. Taddei's American debut took place at San Francisco in 1957 where his Macbeth was welcomed as both well-sung and insightfully characterized. Similar praise was awarded his Scarpia. Chicago heard Taddei for the first time in 1959 when his Barnaba menaced Eileen Farrell's Gioconda. London heard him at Covent Garden in the 1960s when he presented his Rigoletto, Macbeth, and Iago. His much-delayed Metropolitan Opera debut took place on September 25, 1985, when his Falstaff was embraced by the public and critics alike. Several previous attempts to engage him had been made by the Met, but requests for auditions the singer felt unnecessary, the wrong fees, the wrong timing and the wrong roles being offered had kept him away. Taddei repeated his plump knight two years later, once again demonstrating a superb theatrical sense and barely diminished vocal resources.