structural:expansion, quality:corruption
New York Philharmonic conducting debut

Bernstein would later make his New York Philharmonic conducting debut. On November 14, 1943, having recently been appointed assistant conductor to Artur Rodziński of the New York Philharmonic, Bernstein made his major conducting debut at short notice—and without any rehearsal—after guest conductor Bruno Walter came down with the flu. The challenging program included works by Robert Schumann, Miklós Rózsa, Richard Wagner, and Richard Strauss.

The next day, The New York Times carried the story on its front page and remarked in an editorial, "It's a good American success story. The warm, friendly triumph of it filled Carnegie Hall and spread far over the air waves." Many newspapers throughout the country carried the story, which, in combination with the concert's live national CBS Radio Network broadcast, propelled Bernstein to instant fame. Over the next two years, Bernstein made conducting debuts with ten different orchestras in the United States and Canada, greatly broadening his repertoire and initiating a lifelong frequent practice of conducting concertos from the piano.

On January 28, 1944, Bernstein conducted the premiere of his Symphony No. 1: Jeremiah with the Pittsburgh Symphony Orchestra and soloist Jennie Tourel. In the fall of 1943, Bernstein and Jerome Robbins began work on their first collaboration, Fancy Free, a ballet about three young sailors on leave in wartime New York City. Fancy Free premiered on April 18, 1944, with the Ballet Theatre (now the American Ballet Theatre) at the old Metropolitan Opera House, with scenery by Oliver Smith and costumes by Kermit Love.

Bernstein and Robbins decided to expand the ballet into a musical and invited Comden and Green to write the book and lyrics. On the Town opened on Broadway's Adelphi Theatre on December 28, 1944. The show resonated with audiences during World War II, and it broke race barriers on Broadway: Japanese-American dancer Sono Osato in a leading role; a multiracial cast dancing as mixed race couples; and a Black concertmaster, Everett Lee, who eventually took over as music director of the show. On the Town became an MGM motion picture in 1949, starring Gene Kelly, Frank Sinatra, and Jules Munshin as the three sailors. Only part of Bernstein's score was used in the film and additional songs were provided by Roger Edens.

From 1945 to 1947, Bernstein was the music director of the New York City Symphony, which had been founded the previous year by the conductor Leopold Stokowski. The orchestra (with support from Mayor Fiorello La Guardia) had modern programs and affordable tickets. In 1946, Bernstein made his overseas debut with the Czech Philharmonic at the inaugural Prague Spring International Music Festival. He also recorded Ravel's Piano Concerto in G major as soloist and conductor with the Philharmonia Orchestra. On July 4, 1946, Bernstein conducted the European premiere of Fancy Free with the Ballet Theatre at the Royal Opera House in London. On August 6 he conducted opera professionally for the first time, with the American premiere of Benjamin Britten 's Peter Grimes at Tanglewood, commissioned by Koussevitzky. That same year, Arturo Toscanini invited Bernstein to guest conduct two concerts with the NBC Symphony Orchestra, one of which featured Bernstein as soloist in Ravel's Piano Concerto in G major.