In 1916, after a short period as a World War I soldier, he was appointed professor of art history at the University of Freiburg in the department Wilhelm Vöge had created.
During his Freiburg years, he became a friend of Ernst Buschor, Ludwig Curtius, Edmund Husserl and Martin Heidegger.
In 1925 he published a book on German sculptors of the thirteenth century and two years later his groundbreaking essay, "Über den gotischen Kirchenraum", which introduced the term "diaphane Struktur" into discussions about Gothic architecture.
[3] As he sympathized with Nazi ideology for some time,[4] he was dismissed from teaching after World War II, but reinstated shortly thereafter.
Jantzen's students included, among others, Kurt Bauch, Martin Gosebruch, Julius S. Held, Robert Oertel, Willibald Sauerländer and Paul Wescher.