As one of the chief representatives of French music in the early twentieth century, Nadia Boulanger is typically ignored in discussions of the reception of Gustav Mahler’s music, which—like most studies of reception—focus primarily on press accounts and public events. Moreover, Boulanger is usually considered in the context of a broader French aversion, in the first half of the twentieth century, to Mahler’s late-Romantic Austro-German idiom. But a range of documentary evidence concerning her attendance at the 1920 Mahler festival in Amsterdam, including previously unexamined correspondence as well as scores annotated in her hand, reveals that, motivated by a post-World War I spirit of internationalism, Boulanger contributed materially to the study and performance of Mahler. She encouraged audiences to consider his music’s emotional power and analyzed it in a way that drew attention to its orchestration and the horizontal aspects of its construction. She also introduced such figures as Aaron Copland to Mahler’s music, preparing him to approach it in a way that centered on the vocabulary of neo-classicism. Boulanger’s engagement with Mahler not only contributes to our picture of the composer’s reception, but also reveals the historiographical value of discourses that take place behind the scenes.