Kastler, who was born in Gebweiler (now Guebwiller in France), was educated at the Ecole Normale Supérieure. He then taught at the University of Bordeaux where he became professor of physics in 1938. He moved to the University of Paris in 1941 where he remained until his retirement in 1972.

He worked on double-resonance techniques of spectroscopy, using absorption by both optical and radiofrequency radiation to study energy levels in atoms. He also introduced the technique known as ‘optical pumping’ – a method of exciting atoms to a different energy state. In practical terms Kastler's work led to new frequency standards and new methods for the measurement of weak magnetic fields. Kastler received the 1966 Nobel Prize for physics for his work on double resonance.