He was active with a post in Athens and at the Belgian Permanent Representation to the European Union.
He worked in the cabinets of Prime Ministers Herman Van Rompuy and Yves Leterme (CD&V) and in 2011 became Head of Cabinet of Minister of Foreign Affairs Steven Vanackere (CD&V).
When Koen Geens succeeded Vanackere in the Di Rupo Government and Chief of Staff Eric Kirsch left for the OECD, he became Chief of Cabinet of Finance.
In 2014, in the Michel I Government, Geens was given the Justice Department, and he re-elected Houssiau as head of his political cabinet.
Since the summer of 2017, he has been chief of staff to King Philippe, succeeding Frans van Daele.