Born in Copenhagen into an artistic family, his father was director of the Royal Danish Theatre. He studied art in Paris from 1904 to 1911, and then lived in England from about 1911 to 1916. He received his first English commission by Hodder and Staughton to illustrate a collection of fairytales, providing 24 colour plates – In Powder and Crinoline, Fairy Tales Retold by Sir Arthur Quiller-Couch, 1913. He also provided illustrations for the childrens’ collection East of The Sun West of The Moon, old tales from the North, 1914.Meanwhile he painted (in tempera) landscapes in the Dover area. Nielsen came into contact with The Society of Tempera Painters where he learned new skills, and was able to reduce the time involved in the process. In 1917 he left for New York where there was an exhibition of his work. Together with a collaborator, Johannes Poulsen, he painted stage scenery for the Royal Danish Theatre in Copenhagen. In 1939 he left for California and worked for Hollywood companies. His art has a very hard edge, which makes them suitable for conversion into cinema cartoon form. In the 1930s went to work for The Walt Disney Company, where his work was used in the “Ave Maria” and “Night on Bald Mountain” sequences of Fantasia. In 1940 he was laid off. He did some work on “The Little Mermaid” story, but it was almost 50 years before this reached the cinema. His final years were spent in poverty. His last works were for local schools and churches. His reputation has grown steadily since then.
 
Bron: en.wikipedia.org