As was typical in France at this time, university professors began their careers in the provinces and if they were successful enough could be promoted to a chair in Paris. Lions followed this pattern when he was appointed Professor in the Faculty of Science of the University of Paris in 1963.
Bilang ay karaniwang sa France sa oras na ito, university professors nagsimula ang kanilang propesyon sa probinsya at kung sila ay matagumpay sapat na maaaring itaas sa isang upuan sa Paris. Lions sinundan ito pattern kapag siya ay hihirangin Propesor sa Faculty of Science ng University of Paris sa 1963.