Leonard Bernstein’s advocacy of Gustav Mahler’s music has been viewed primarily as a consequence of his position in a lineage of prominent conductors who preceded him as Mahler proponents. But treating Bernstein’s conducting as divorced from another crucial aspect of his musical life—his composing—has limited our understanding of how ideas about music circulated in the twentieth century. This study argues that Bernstein’s relationship with Aaron Copland, his mentor as a composer, played a central role in his engagement with Mahler’s music. Correspondence suggests that from the 1930s, Copland discussed Mahler’s compositions with Bernstein and supported his interest in performing them, particularly the Second Symphony. A script for a lecture from the New York Philharmonic Mahler festival in 1960 shows that Bernstein borrowed content and phraseology from Copland’s 1941 book Our New Music in an attempt to define Mahler as a forerunner of twentieth-century tonal composition.