Under the law of 24 February 1875 on the organization of the Senate, there were 300 members of whom 225 were elected by the departments and colonies, and 75 were elected by the National Assembly. The 75 were elected by list and by an absolute majority of votes, and were irremovable, like the members of the Chamber of Peers under the Bourbon Restoration and the July Monarchy.

If a senator for life died or resigned, the Senate would elect a replacement within two months.^[1] By the law of 10 December 1884 appointment of immovable senators ceased and the immovable senators gradually disappeared.^[1] Émile Deshayes de Marcère, the last surviving sénateur inamovible, died in 1918.^[1] Overall there were 116 lifetime senators.^[2] The first 75 had been appointed by the National Assembly and the remaining 41 by the Senate itself.^[1]

Notable immovable senators included Gaston Audiffret-Pasquier, first president of the Senate; the scientist Marcellin Berthelot, who became minister of public education and then minister of foreign affairs; Monseigneur Dupanloup; Jules Grévy, elected President of the Republic in 1879; Louis Martel, elected President of the Senate in 1879; Elie Le Royer, elected President of the Senate in 1882; Auguste Scheurer-Kestner, the defender of Alfred Dreyfus; the abolitionist Victor Schœlcher and the statesman Henri-Alexandre Wallon.^[1]

In 2005, there was questioning about the status of former Presidents of the Republic. According to the constitution of the Fifth Republic, former presidents are de jure members of the Constitutional Council, which poses a problem of possible partiality. Some members of Parliament and commentators suggested that it should be replaced by a life membership in the Senate.^[3][a] This proposal was, however, not enacted.

The senators for life were:^[2]