Organised crime was "flourishing" in some places because many of the resources the justice system needed to fight it were being drawn off to counter terrorism, he warned this week.
Mr Mehlis is the senior public prosecutor and head of the international affairs section in the Office of the Attorney-general in Berlin.
He has been a senior public prosecutor since 1992 and, during his career, has prosecuted terrorism and organised crime cases.
Among his many cases, he proved the involvement of the terrorist Carlos and Syrian diplomats in a terrorist attack on the French Culture Centre in Berlin in 1983.
In 2005, the then United Nations secretary-general, Kofi Annan, appointed Mr Mehlis as commissioner of the United Nations International Independent Investigation Commission into the assassination of the former Lebanese prime minister Rafiq al-Hariri and 22 other people in Beirut.
Mr Mehlis gave a public talk at the University of Otago on "International Justice and Terrorist Crimes".
"Sometimes we get the impression that terrorism is the only international challenge to democracies, which is not true," he said in an interview.
The threat posed by organised crime, including drug dealing, prostitution and people-trafficking, was as important as terrorism.
Within European Union countries, even taking into account the recent train bombings in Madrid and London, organised crime had, over the years, caused more damage than terrorism, he said.
His public talk was in association with the Dunedin branch of the Institute of International Affairs, and his visit was also supported by the Otago politics department and faculty of law.
