She soon developed an interest in music composition but her early ventures into this area were strongly discouraged by both her teacher and her relatively wealthy and conservative family, who viewed the pursuit as an eccentric hobby that was "too exciting for a delicate child".

[1] In 1936, Pentland entered the graduate music program at the Juilliard School in New York City where she studied 16th-century counterpoint with Frederick Jacobi and modern composition techniques with Bernard Wagenaar through 1939.

During these years, her own compositions took on a language that was primarily neoclassical, showing the influence of Paul Hindemith, Igor Stravinsky, and later Aaron Copland (with whom she studied at the Tanglewood Music Center during the summers of 1941 and 1942).

It is the work of this period which is regarded as her finest, being described by musicologist David Gordon Duke as music that "drew on the textures and organizational principles of the Webern school but was suffused with a lyricism that was expressly individual".

[7] Pentland's centennial was celebrated with a 2012 concert series sponsored by the Canadian Music Centre (BC Region), and with a revival of her opera The Lake presented by Astrolabe Musik Theatre, the Turning Point Ensemble, and Westbank First Nation.