As a younger son, Simon de Montfort attracted little public attention during his youth, and the date of his birth remains unknown. He is first mentioned when his mother made a grant to him in 1217. As a boy, Montfort accompanied his parents during his father's campaigns against the Cathars. He was with his mother at the Siege of Toulouse in 1218, where his father died after being struck on the head by a stone pitched by a mangonel. In addition to Amaury, Simon had another older brother, Guy, who was killed at the siege of Castelnaudary in 1220. As a young man, Montfort probably took part in the Albigensian Crusades of the early 1220s. He and Amaury both took part in the Barons' Crusade.