While still a student, two of his early compositions were publicly performed: Die vier marianischen Antiphonen at the Lower Rhineland Festival in Düsseldorf in 1928, and his First String Quartet in Königsberg in 1930 (Weber 2001).

In 1931 he completed his studies with the State Exam for a high teaching office, after he accepted a lectureship in music theory at the Hochschule für Kirchenmusik Heidelberg.

[citation needed] Fortner moved to the Heidelberg Kohlhof and there a group of very young students formed around him, who showed interest in the modern music of 1933.

[citation needed] In 1946 he joined the circle of the Darmstadt Internationale Ferienkurse für Neue Musik, and taught within that framework.

Together with eleven other composer-friends (Conrad Beck, Luciano Berio, Pierre Boulez, Benjamin Britten, Henri Dutilleux, Alberto Ginastera, Cristóbal Halffter, Hans Werner Henze, Heinz Holliger, Klaus Huber, and Witold Lutosławski), he was asked by Russian cellist Mstislav Rostropovich, on the occasion of the 70th birthday of the Swiss composer and art patron Paul Sacher, to write a composition for cello solo using the notes of his name (eS, A, C, H, E, Re).