In 1986, he graduated in Law with honors, writing a thesis on time-sharing. For two years, he worked at his father’s law firm while attending classes led by President Giulio Capozzi in Naples.

In 1989, he joined the legal advice staff of Cariplo, where he specialized in corporate law and the regulation of financial markets. He was involved in a large number of operations, most notably the first concurrent public takeover bid (RoLo). He took part in work groups created by ABI (Associazione Bancaria Italiana, Italy's Bank Association) for regulation compliance on brokerage firms and corporate governance. 

From 1998 to 2008, he was Legal Affairs Manager at the Borsa Italiana, while for several years overseeing the Listing Department. With full autonomy, his team managed delicate investigative cases (listings, delistings, sanctions) as well as the development of the group through takeovers until the group itself was integrated into the London Stock Exchange.

He participated in the writing of the Codice di Autodisciplina (self-discipline regulation) for listed companies (1999, and subsequent editions) as well as the Framework of corporate governance for Elite (2018). He is presently Secretary of the Comitato per la Corporate Governance (Committee for Corporate Governance) and is a member of the Technical Secretary.

In 2008, he opted to devote himself to freelance work, working with prominent national and international law firms. Accordingly, he has dealt mostly with equity capital market operations (IPO's and capital increases), corporate reorganization, consultancy for corporate law and financial markets, sanction procedures in market abuse, and other regulation infringements.

In 2016, he and Gianluca Grea founded CG Partners, a law firm-boutique that subsequently merged into Koiné.

He has held positions at several listed and non-listed companies.

Mr. Chieffi is the author of several publications on corporate matters, and he serves as a lecturer at conferences and seminars.