Prior to his international appointments, he served as Deputy Prosecutor, then Chief Deputy Prosecutor at the Court of First Instance in Eupen (Belgium) from 1989 to 1997, before becoming Deputy to the Prosecutor-General at the Liège Court of Appeal. Mr. Brammertz was first a national magistrate then the head of the Federal Prosecution of the Kingdom of Belgium.
In September 2003 he was elected by the Assembly of State Parties as the first Deputy Prosecutor of the International Criminal Court. In that capacity, he was in charge of establishing the Investigations Division of the Office of the Prosecutor, and initiated the first ICC investigations in Uganda, the Democratic Republic of Congo and Darfur. In January 2006 United Nations Secretary-General Kofi Annan appointed him as Commissioner of the United Nations International Independent Investigation Commission into the assassination of former Lebanese Prime Minister Rafik Hariri, a post he held until the end of 2007, when he was appointed Chief Prosecutor of the International Criminal Tribunal for the former Yugoslavia, where he served until its closure at the end of 2017. Since 2016, he served as a Chief Prosecutor of the International Residual Mechanism for Criminal Tribunals.