Jean Casimir-Perier was born in Paris on 8 November 1847, the son of Auguste Casimir-Perier, the grandson of Casimir Pierre Perier, premier of Louis Philippe, and the great grandson of Claude Périer, one of the founders of the Bank of France.

He entered public life as secretary to his father, who was Minister of the Interior under the presidency of Thiers.

In spite of the traditions of his family, Casimir-Perier joined the group of Republicans on the Left, and was one of the 363 on the Seize-Mai (1877).

He refused to vote the "expulsion of the Princes" in 1883, and resigned as Deputy upon the enactment of the law (26 June 1886) because of his personal connections with the family of Orléans.

Casimir-Perier explained his action by the fact that he found himself ignored by the ministers, who did not consult him before taking decisions, and did not keep him informed upon political events, especially in foreign affairs.