He was educated at a Jesuit school, studied law at Paris, and became one of the editors of the Revue de Bruxelles.

As such he took historic initiatives to promote the Dutch language that had lost ground in political life since the Belgian Revolution of 1830 (against the Union with Holland as the United Kingdom of the Netherlands) even though the majority of the population spoke Dutch.

He attempted, by combining the moderate elements of the Catholic and Liberal parties, the impossible task of resolving the educational and other questions then dividing Belgium.

He became involved in financial speculations which lost him his good name as well as the greater part of his fortune; and, though he was never proved to have been more than the victim of clever operators, when in 1871 he was appointed by the Catholic cabinet governor of Limburg, the outcry was so great that he resigned the appointment and retired definitively into private life.

De Decker, who was a member of the Belgian academy, wrote several historical and other works of value, of which the most notable are: