In 1920 Kastler went to Paris to study at the École Normale Supérieure. After serving on the science faculties at Bordeaux and Clermont-Ferrand in France and Leuven (Louvain) in Belgium, he returned to the École Normale Supérieure to teach (1941–68). He was professor and codirector of the laboratory of physics there at the time of the Nobel award. During his long and fruitful teaching career he trained an entire generation of French physicists. From 1968 until his retirement in 1972, Kastler served as director of research at the National Centre of Scientific Research. He was active in peace movements and in groups opposed to nuclear proliferation.

Kastler's Nobel Prize-winning research facilitated the study of atomic structures by means of the radiations that atoms emit under excitation by light and radio waves. His method of stimulating atoms in a particular substance so that they attain higher energy states was called “optical pumping.” (optical pumping) Since the light energy used to stimulate the atoms was reemitted, optical pumping marked an important step toward the development of the maser and the laser.