Horst was the musical director for the Denishawn company (1916-25) before working as musical director and dance composition teacher for Martha Graham's school and dance company (1926-48).

One memorable piece of advice that Horst gave dancers in his lessons in the 1930s, at times delivered in a sarcastic tone: "when in doubt, turn.

[7] Apart from being a personal friend and mentor to Graham, Horst worked and wrote scores for many other choreographers, including: Horst composed scores for the Denishawn company, including Japanese Spear Dance (1919).

He composed several of Graham's early group works: Primitive Mysteries (1931), Celebration (1934), Frontier (1935), and El Penitente (1940).

[...] On a later tour, there was a time, according to Agnes de Mille, when Miss Ruth passed by the piano, which was on stage, asking Louis how much longer she had to go.