Born at the end of the 19th century in Bayonne, France, as a son of a Jewish merchant. He was wounded while serving in the French army during World War I. The experience marked him for life and shaped his anti-war, humanitarian attitude.
In the inter-war years, he worked as a law professor in Paris and then represented France at the League of Nations (1924–1938). In that period, he focused his efforts on supporting disarmament initiatives. During World War II, he joined Charles de Gaulle in London and became a member of the French government in exile.
He is famous for drafting, together with Eleanor Roosevelt, the text of the Universal Declaration of Human Rights, the most fundamental document in the international human rights protection system. On 10 December 1948, the Declaration was adopted by the General Assembly by a unanimous vote. Cassin advocated recognition of the Declaration, which has the formal status of a non-binding resolution, as a document of legal significance. Today, in the opinion of most lawyers, the Declaration has in fact become an integral element of customary international law.