Leon is the son of Theodore Rienach and Fanny Therese Kann. He was a musical composer and came from an illustrious family of art, science, and statesmen. He was a member of the Institut de France. His father was also a member and upon his death, he left the Villa Kerylos (a Greek-styled property built in 1908) to the Institut. Leon became the keeper of the archvies at Villa Kerylos.
He married Beatrice de Camondo in March 1919 (they were later divorced or separated) and they are the parents of the following:
1) Fanny (1920-1943)
2) Bertrand (1923-1943)
After the Nazi invasion of Paris, Leon wrote a letter to Beatrice asking her to leave Paris with their two children, which she ignored. Leon, Beatrice, Fanny, and Bertrand were deported to Auschwitz concentration camp, on convoy 62. They all perished at Auschwitz in 1943.