Crespin was a student of Janlet and Garin-White in Brussels, and later, Bonnat in Paris. During his early studies he collected Japanese art works that for him created an awareness of quality art.
After marrying at 30, he became a professor of art in Schaerbeek, where he met Paul Hankar, an architect, and also a professor. They begin collaboration on a number of architectural projects. He along with other notable Belgian artists were asked by King Leopold to utilize the materials of the Belgian Congo as part of their design for an exhibition hall at Atervueren. This captured the imagination of the Belgian people, and was useful to King Leopold, thus furthering Crespin's career.
With Paul Hankar, Crespin asked the committee organizing the Exposition Universelle of 1897 in Brussels to create a modern section, but this was not done. Crespin began a collaboration with the portrait painter, Edouard Duyck (1856-1897). Together they created a number of decorations and costumes and posters for the theater, as well as other commercial works.
Most importantly, from around 1886 or 1887, aided by Duyck, Crespin was the first in Belgium, (probably along with Armand Rassenfosse) to create such an interest in posters. His actions, along with those of foreign posterists such as Cheret, helped to stimulate the industry of printing in Belgium.
L'Affiche Belge 1892-1914, Catalogue by Yolande Oostens-Wittamer, Bruxelles: Bibliothèque Royal Albert 1er, 1975.