Dr Wallner was appointed Prosecutor General of the Principality of Liechtenstein in 2000. As the Head of the Public Prosecutor's Office (PPO), he is in charge of all criminal investigations and prosecutions. He reorganised the PPO and built up a network of international contacts. He also made asset seizure and forfeiture a priority in the work of his office. Liechtenstein became one of the first civil law jurisdictions to successfully conclude major (civil)-forfeiture cases.

Dr Wallner is an expert in the Stolen Asset Recovery program of the World Bank and the United Nations. He assists the government of Liechtenstein in drafting new legislation in the field of criminal law, money laundering, due diligence, mutual legal assistance and administrative assistance including the exchange of information in tax matters. He served in numerous delegations negotiating bilateral and multilateral treaties like the Mutual Legal Assistance Treaty with the United States, the association of Liechtenstein to Schengen and Dublin and the Anti-Frauds-Agreement with the European Union.

Dr Wallner is also a lecturer at the Liechtenstein University and an expert in sports law. He has published articles on liability in mountain sports. He is the Vice-President of the Austrian Society for Alpine Safety and serves as the Austrian delegate to the Legal and Safety Committee of the International Skiing Federation. After graduating from Karl-Franzens University in Innsbruck, Austria, he worked as a Richteramtsanwärter at various courts in Austria and was appointed to be a District Court judge in Innsbruck in 1986. In 1988, he became a Public Prosecutor at the Prosecutors Office for the Province of Tyrol, Austria.