He was educated at a Jesuit school, studied law at Paris, and became one of the editors of the Revue de Bruxelles. In 1839 he was elected to the Belgian lower chamber, where he gained a great reputation for oratory. He was a member of parliament from 1839 to 1866. 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. De Decker was at the origin of a 'Petition in favour of the Flemish language' in 1840 and of the setting up of a 'Committee on Flemish Grievances' in 1855.
