Hagemann grew up in a middle-class family in Essen, where he attended the humanist Gymnasium am Burgplatz.
In the second decade of the 20th century, first under the influence of his friend Ernst Gosebruch, the director of the Essen Museum of Art, Hagemann turned toward painters of Die Brücke and Emil Nolde.
A significant highlight of the pictures was Kirchner's "Berlin Street Scene" (Neue Galerie New York).
After his accidental death in 1940, he left a collection of about 1,900 art objects, including nearly a hundred paintings by Ernst Ludwig Kirchner, Karl Schmidt-Rottluff, Erich Heckel, Otto Mueller, Emil Nolde and others.
Today, pictures from Hagemann's collection are found in museums around the world, and some of them are privately owned.