He had two brothers, Gert Marcus (1914–2008; a noted Swedish artist and sculptor, and a recipient of the Prince Eugen Medal), and Holger, and one sister, Anna-Britta.

Dahl left Germany as the Nazi Party was coming to power and continued his studies at the University of Zurich, along with Volkmar Andreae and Walter Frey [de].

He served as a vocal coach and chorus master for the world premieres of Alban Berg's Lulu and Paul Hindemith's Mathis der Maler.

He had a varied musical career as a solo pianist, keyboard performer (piano and harpsichord), accompanist, conductor, coach, composer, and critic.

[clarification needed][7] He also worked in the entertainment industry, touring as pianist to Edgar Bergen and his puppets in 1941 and later for comedian Gracie Fields in 1942 and 1956.

[9] He performed on keyboard instruments in the soundtrack orchestras for many films at Fox, Goldwyn Studios, Columbia, Universal, MGM, and Warner Bros., as well as the post-production company Todd-AO.

[11] Dahl conducted the soundtrack to The Abductors (1957) by his pupil Paul Glass[12] and performed both second and third movements of Beethoven's Pathétique Sonata in the 1969 animated film A Boy Named Charlie Brown.

[19] In 1945 he joined the faculty of the University of Southern California Thornton School of Music in Los Angeles, where he taught for the rest of his life.

She accepted his homosexuality, helped him to keep it hidden, and shared his affection with a lover that Dahl had met on a trip to Boston, and occasionally visited there.

[27][28] He maintained an intimate, though never exclusive, relationship for the last fifteen years of his life with Bill Colvig, whom he met on a Sierra Club hiking trip.

[33] He assessed the relationship between Dahl's private and public sides in these words:[34] His social life and his compositions never seemed to acquire that ease of communication that sustain many gifted creators, those titans whose ability to tap into the well-springs of their being allow them to produce a copious and enviable body of artistic endeavor.

[38] Recently there has been a revival of interest in the history of the Marcus–Dahl family, its flight from Hamburg, and the cultural contributions of Ingolf Dahl and his brother, the sculptor Gert Marcus.