He was responsible for the accession negotiations of Great Britain, the Republic of Ireland, Denmark and Norway, and for assistance to developing countries.

In 1973, he entered the government of Pierre Messmer as Secretary of State for Coopération, and was then named Secretary of State to the Ministry of Agriculture and Rural Development in the government formed by Jacques Chirac after the election of Valéry Giscard d'Estaing to the presidency of the French Republic in 1974.

Deniau became France's ambassador to Madrid, on the request of the new king Juan Carlos, with whom he had begun a friendship during regattas.

Deniau would play an active advisory role to the king and the government during Spain's democratic transition following the death of General Franco.

In September 1977, Jean-François Deniau was named Secretary of State to the Ministry of Foreign Affairs in the government of Raymond Barre, then Minister of Foreign Commerce (1978), and finally Minister of Administrative Reform in Raymond Barre's last government (1981).