In 1958, he became the director of Foreign Relations for the European Commission. He was the author of the foreword of the Treaty of Rome. In 1963, he was named French ambassador to Mauritania and in 1967 he was appointed as one of the French European Commissioners, as a member of the Rey Commission, in 1970 followed by his membership of the Malfatti Commission. He was responsible for the accession negotiations of Great Britain, the Republic of Ireland, Denmark and Norway, and for assistance to developing countries.