Cantelli was both the youngest and shortest-lived of the world-class conductors born between 1908 and 1920, a remarkable group that included Karajan, Solti, Leinsdorf, Giulini, and Bernstein. His musical youth was conventional: early evidence of a gift, keyboard instruction, his first piano recital at 14, etc. Slonimsky wrote that he played in his father's military band. Cantelli entered the nearby Milan Conservatory, where he majored in conducting and composition (under Giorgio Ghedini). He returned to his home city in 1943 as director of the Theatre Coccia, opened in 1888 by Toscanini, who became his champion in the last decade of both their lives. However, Cantelli was forced to join the Italian army despite his outspoken loathing of Nazism. For this he was interned in a German labor camp near Stettin until illness finally required hospitalization at Bolzano. He escaped with a forged passport and lived in Milan under an assumed name until Fascist troops took him hostage. Following the liberation, Cantelli was freed to pursue conducting engagements. The La Scala Orchestra became his first, with Claudio Abbado's father as violin soloist. Operatic and concert engagements followed, first in Italy, then elsewhere in Europe including Budapest and Vienna. Within three seasons he proved himself deserving of the mercurial career that Toscanini launched.
The "Old Man" began his ninth decade looking for a younger associate to keep the NBC Symphony Orchestra (created for him in 1938) on course during his absences. La Scala's intendant, Antonio Ghiringhelli, took him to an off-season concert by Cantelli. Before it was half-over, Toscanini whispered "that is me directing this concert!" He arranged for the young conductor's immediate NBC debut on January 15, 1949. Afterwards, Time magazine featured a profile likening him physically to Frank Sinatra, but musically to Toscanini. Until NBC disbanded the orchestra in 1954, Cantelli conducted annually, beginning with four but expanding to eight programs. In 1951 he made the first of five annual appearances as a regular guest-conductor of the NY Philharmonic along with Bruno Walter and Szell. He recorded Vivaldi's Seasons with them for Columbia, but RCA and EMI owned his services contractually. With NBC he recorded four performances, with Walter Legge's London Philharmonia considerably more, a few in stereo.
Toscanini's endorsement proved a double-edged sword, however. Some NYP players echoed the complaint by two daily newspaper critics that the regular guests conducted too much standard repertory, perhaps because Cantelli was being groomed to take over. (With Mitropoulos on his way out, Bernstein would serve as a stopgap until Cantelli was "ready." Bernstein had lost Boston in 1949 because he was Jewish; in NYC the stigma was his Broadway career.) Cantelli had been a taskmaster who rehearsed and conducted without score; sluggish players resented him — so openly that he asked without success to be released from a late November 1956 engagement. La Scala formally named him music director on November 16 (to succeed Giulini, who had succeeded Victor de Sabata in 1953 but detested administrative duties). One week later, a Lineo Aereo Italiano plane from Milan to NYC crashed following a stopover at Orly Airport near Paris. Guido Cantelli was not among the survivors.
Toscanini died two months later without being told of Cantelli's death. Legge wrote in a memorial tribute to Cantelli, "no other conductor in the history of the art has established, so early in life, so wide a fame." While studio recordings validate that encomium, none quite captured the incandescence of his live performances. One of a kind, unmatched since, Cantelli was a supernova.