In 1974, Sandri became an official of the Apostolic Nunciature in Madagascar and Mauritius, which also serves as the Apostolic Delegation in the islands of Comoros and Réunion in the Indian Ocean. He then served in the Vatican Secretariat of State, as secretary of the Substitute for General Affairs, including future cardinals Eduardo Martinez Somalo and Edward Cassidy from 1977 to 1989; and in the Apostolic Nunciature in the United States as permanent observer of the Holy See before the Organization of American States, from 1989 to 1991.