Marnold was the co-founder of the Mercure musical and a critic on the newspaper Mercure de France. He was a keen supporter of Ravel's music, and became a close friend.
In 1905, Marnold wrote an article entitled "Le scandale du Prix de Rome", in which he challenged the decision of the jury which had eliminated Ravel from the competition in the first round. Was it conceivable, he asked, that the composer of Jeux d'eau and the Quatuor à cordes was unworthy of reaching the final? And he pointed out that the six chosen finalists were all pupils of the same teacher, Charles Lenepveu, who happened also to be on the jury for the Prix de Rome. (Mercure de France, 1.vi.1905, pp.466-469).
Ravel dedicated to Marnold Le gibet, the second movement of Gaspard de la nuit. In a letter, he wrote:
"J'ai l'intention de vous dédier "Le gibet". Ce n'est pas que j'estime que méritiez la corde pour vous pendre mais c'est le moins difficile des trois. Vous pourrez le jouer, ce que, soît dit sans amoindrir vos qualités de virtuose, je n'oserais espérer des deux autres.... MR". ( Chalupt, , p.71).
It was Marnold's daughter Georgette that Ravel asked in 1920 to help him find a house outside Paris, which turned out to be Le Belvédère at Montfort L'Amaury. (Chalupt, , letter 25.iii.1920).