[3] Metzner became interested in law after reading Franz Kafka and a newspaper story about a shepherd with limited language skills who was sentenced to death.
Metzner was a recognized specialist in criminal law cases, particularly known for his ability to detect defects in procedure and freeing clients on technicalities.
[4] Metzner represented the interests of many individuals or companies in court; Loïk Le Floch-Prigent, Jean-Marie Messier, Jacques Crozemarie, Timothy Koogle CEO of Yahoo!, Patrick Puy former CEO of Moulinex, the Bouygues group, Continental Airlines for the Air France Flight 4590 crash, the company that issued RINA certificates of seaworthiness of the Erika, Pierre-Yves Gilleron, a defendant in the Elysée wiretapping scandal, Bertrand Cantat, Florent Pagny, Jérôme Kerviel, Dominique de Villepin, Nike, Françoise Bettencourt-Meyers, and former Panamanian president, Manuel Noriega.
In April 1994, he was the lawyer for Miss Luong Kwan Yin (Tong Yen Restaurant) which became the case of the French Republic – 17 May 2002 – in the Swiss Federal Court in Lausanne.
Metzner was also involved in cases at the request or on behalf of the RPR, PS, PC, Corsican separatists, the Church of Scientology, the Council of the College of Physicians, in the case of "fake voters of the third arrondissement of Paris" and a group of French citizens jailed in the Dominican Republic for cocaine trafficking.