Bull was an important and widely-praised nature artist of his era. He illustrated books by Jack London and Rudyard Kipling, as well as many other authors now less well-remembered, like Charles G. D. Roberts and Hal George Evarts. He also wrote animal stories of his own. Bull provided both text and pictures for Under the Roof of the Jungle: A Book of Animal Life in the Guiana Wilds (1911). In addition to books, Bull did extensive magazine work, including covers for the Saturday Evening Post.
Because his father had thought that art was a waste of time, Bull learned taxidermy as a trade — which enhanced his understanding and appreciation of animal forms.