Carlo Trezza joined the Italian diplomatic service in 1970. He served in Zambia, Israel and Germany before returning to Rome in 1981 to work at the Ministry of Foreign Affairs within the Directorate General for Political Affairs. In 1984, he was appointed First Counsellor at the Embassy of Italy in Madrid. In 1989, he became Head of the Press and Information Office at the Embassy of Italy in Washington, a position he held for three years. From 1992 to 1996, he served as Head of the Department for European Security and Disarmament at the Ministry of Foreign Affairs in Rome. In 1997, he was assigned to the Office of the Secretary General of the Ministry of Foreign Affairs.