Kastler was born in Guebwiller (Alsace, German Empire) and later attended the Lycée Bartholdi in Colmar, Alsace, and École Normale Supérieure in Paris in 1921. After his studies, in 1926 he began teaching physics at the Lycée of Mulhouse, and then taught at the University of Bordeaux, where he was a university professor until 1941. Georges Bruhat asked him to come back to the École Normale Supérieure, where he finally obtained a chair in 1952.