Born Leonard Warenoff in New York to Russian immigrant parents, Warren was first employed in his father's fur business. In 1935, he joined joined the chorus at Radio City Music Hall. In 1938, he entered the Metropolitan Opera Auditions of the Air. Despite the fact Warren was obviously a novice, his natural gifts were apparent, and he was immediately given a contract. The Met sent him to Italy that summer with a stipend to study.
Returning to America, Warren made his concert debut at the Metropolitan Opera in excerpts from La Traviata and Pagliacci during a concert in New York in November 1938. His formal operatic debut took place there in January 1939, when he sang Paolo in Simon Boccanegra.
Warren later sang in San Francisco, Chicago, Mexico City, and Buenos Aires, he appeared at La Scala in Milan in 1953, and in 1958, he made a highly successful tour of the Soviet Union, but for most of his career he remained in New York and sang at the Met.
Although he sang Tonio in Pagliacci, Escamillo in Carmen, and Scarpia in Tosca, he was particularly acclaimed as one of the finest interpreters of the great Verdi baritone roles, above all the title role of Rigoletto, which was captured in 1950 in an electrifying recording with soprano Erna Berger and tenor Jan Peerce, conducted by Renato Cellini.
His last complete performance at the Met was as Simon Boccanegra on March 1, 1960. Three days later, in a performance of La Forza del Destino with Tebaldi, Warren was about to launch the vigorous cabaletta to Don Carlo's aria, which begins "Morir, tremenda cosa" (to die, a momentous thing), when he pitched face-forward to the floor. A few minutes later he was pronounced dead of a massive cerebral vascular hemorrhage, and the rest of the performance was canceled. Warren was only forty-eight.
Warren was known as a person of an intractable character, who always tried to impose his will on stage designers, managers, and even conductors, in matters of production, direction, and tempi. He caused pain, a colleague once noted, but he had a great voice.
In his book The American Opera Singer (1997), Peter G. Davis wrote of Warren:
The rich, rounded, mellow quality of [Warren's] voice, fairly bursting with resonant overtones, may not have been to every taste, particularly those preferring a narrower baritonal focus that "speaks" more quickly on the note. But by any standards it was a deluxe, quintessentially "Metropolitan Opera sound," one that seemed to take on a special glow and lustrousness as it opened up and spread itself generously around the big auditorium. And of course the easy top was its special glory -- when relaxing with friends Warren would often tear into tenor arias like "Di quella pira" and toss off the high Cs that many tenors lacked. He could have, but never did, overindulge that applause-getting facility.