Montfort was a younger son of Simon de Montfort, 5th Earl of Leicester, a French nobleman and crusader, and Alix de Montmorency. His paternal grandmother was Amicia de Beaumont, the senior co-heiress to the Earldom of Leicester and a large estate owned by her brother Robert de Beaumont, 4th Earl of Leicester, in England.
With the irrevocable loss of Normandy, King John refused to allow the elder Simon to succeed to the earldom of Leicester and instead placed the estates and title into the hands of Montfort senior's cousin Ranulf, the Earl of Chester. The elder Simon had also acquired vast domains during the Albigensian Crusade, but was killed during the Siege of Toulouse in 1218 and his eldest son Amaury was not able to retain them. When Amaury was rebuffed in his attempt to get the earldom back, he agreed to allow his younger brother Simon to claim it in return for all family possessions in France.
Simon arrived in England in 1229, with some education but no knowledge of English, and received a sympathetic hearing from King Henry, who was well-disposed towards foreigners speaking French, then the language of the English court. Henry was in no position to confront the powerful Earl of Chester, so Simon approached the older, childless man himself and convinced him to cede him the earldom. It would take another nine years before Henry formally invested him with the title Earl of Leicester.
Simon de Montfort shared various levels of consanguinuity and "by-marriage" connections with both English and French royal lineages. For instance, his ancestor Simon I de Montfort was father of Bertrade de Montfort who herself was a paternal great-grandmother of King Henry II. He was also descended from Henry I.
As a younger son, Simon de Montfort attracted little public attention during his youth, and the date of 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.