Born in Santa Maria a Vico, Caserta, D'Ambrosio graduated in law in Naples in 1952. In 1957 he was assigned to the Public Prosecutor at the Court of Nola, then he was transferred to the Court of Voghera, and finally in Milan; there D'Ambrosio conducted the criminal investigation of the Piazza Fontana bombing, and was prosecutor in the Banco Ambrosiano trial, among other things. In 1989, he was chosen to direct the department against organized crime and since 1991 he directed the special department covering the crimes against the public administration. In 1992 he entered the mani pulite pool of magistrates which gave him a large notoriety.