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 the Mayor) was aimed at a different audience than the New York Philharmonic, with more modern programs and cheaper tickets. Also in regard to a different audience, in 1945 Bernstein discussed the possibility of acting in a film with Greta Garbo—playing Tchaikovsky opposite her starring role as the composer's patron Nadezhda von Meck.

In addition to becoming known as a conductor, Bernstein also emerged as a composer in the same period. In January 1944 he conducted the premiere of his Jeremiah Symphony in Pittsburgh. His score to the ballet Fancy Free choreographed by Jerome Robbins opened in New York in April 1944 and this was later developed into the musical On the Town with lyrics by Comden and Green that opened on Broadway in December 1944.

On July 4, 1946, Bernstein conducted the European premiere of Fancy Free with the Ballet Theatre at the Royal Opera House in London. In 1946, he conducted opera for the first time, with the American première at Tanglewood of Benjamin Britten's Peter Grimes, which had been a Koussevitzky commission. That same year, Arturo Toscanini invited Bernstein to guest conduct two concerts with the NBC Symphony Orchestra, one of which again featured Bernstein as soloist in the Ravel concerto.

On December 10, 1949, he made his first television appearance as conductor with the Boston Symphony Orchestra at Carnegie Hall. The concert, which also included an address by Eleanor Roosevelt, celebrated the one-year anniversary of the United Nations General Assembly's ratification of the Universal Declaration of Human Rights, and included the premiere of Aaron Copland's "Preamble" with Sir Laurence Olivier narrating text from the UN Charter. The concert was televised by NBC Television Network

He became a well-known figure in the United States through his series of fifty-three televised Young People's Concerts for CBS, which grew out of his Omnibus programs. His first Young People's Concert was televised a few weeks after his tenure began as principal conductor of the New York Philharmonic. He became as famous for his educational work in those concerts as for his conducting. The Bernstein Young People's Concerts were the first and probably the most influential series of music appreciation programs ever produced on television, and they were highly acclaimed by critics. Some of Bernstein's music lectures were released on records; a recording of Humor in Music was awarded a Grammy award for Best Documentary or Spoken Word Recording (other than comedy) in 1961. The programs were shown in many countries around the world, often with Bernstein dubbed into other languages. All of them were released on DVD by Kultur Video (half of them in 2013).