He was educated at the Lycée Fustel-de-Coulanges in Strasbourg before studying at the Faculty of Science in Strasbourg and the Faculty of Science in Paris. He has lectured at many universities and was appointed professor in the Faculty of Science at Grenoble in 1963. He is a member of the French Academy of Sciences.
Jean-Louis Koszul is the cousin of the French composer Henri Dutilleux. Their grandfather, the composer Julien Koszul, was a close friend of Gabriel Fauré. Koszul married Denise Reyss-Brion on July 17, 1948. They had three children: Michel, Bertrand, and Anne.