He was a member of the House of Dampierre, the son of Guy of Dampierre, Count of Flanders and Marquis of Namur, and his second wife Isabelle of Luxembourg.
John was the father of Blanche of Namur, Queen of Sweden and Norway.
He was the elder brother of Guy of Namur, whom he sent to command the Flemish rebels against the French Kingdom in the 1302 Battle of the Golden Spurs.
In September 1290, he was betrothed to Blanche of France, daughter of Philip III.
They were married by contract in Paris on 6 March 1310, confirmed Poissy, January 1313.