Born in Houffalize, John was the eldest son of Margaret II of Flanders by her first husband, Bouchard IV of Avesnes.
[1] Thus, John and his brother Baldwin undertook to receive imperial recognition of their legitimacy and did so from the Emperor Frederick II.
[2] On 5 December 1244, Margaret inherited Flanders and Hainaut and designated her eldest son by her second husband, William III of Dampierre, as her heir.
After two years of fighting, in 1246, Louis IX of France intervened to settle the conflict and granted Hainaut to John and Flanders to William.
[4] Guy was imprisoned and Margaret agreed to sell her rights to Hainaut to Charles of Anjou if he would reconquer it from John.