Klein helped reconstruct French Judaism after Liberation by working with many Jewish organizations. From 1945 to 1950, he served as President of the Union des étudiants juifs de France, of which he was a co-founder. From 1970 to 1973, Klein was Vice-President of the Conseil représentatif des institutions juives de France (CRIF), and was president from 1983 to 1989. He founded the Dîner du CRIF in 1985. He also served on the European Jewish Congress. In 2012, Klein broke his ties with CRIF in a letter to then-President Richard Prasquier, criticizing his response to the Muhammad al-Durrah incident.