 In 1958, he became the chief of staff for Pierre Pflimlin, the last president of the Council of the Fourth Republic before Charles de Gaulle.From 1959 to 1962, he was the chief of staff for prime minister Valéry Giscard d'Estaing, then "chargé de mission" (1962–65), and finally director of Insurances at the Minister of Finance from 1963 to 1967.Poniatowski took a founding part in the Independent Republicans (RI) party, and became an RI deputy for the Val-d'Oise in 1967, as well as the general secretary of the Confederation of the Independents before taking the presidency of the party's successor, the Republican Party, in 1975.