In January 1238, Montfort married Eleanor of England, daughter of King John and Isabella of Angoulême and sister of King Henry III.
While this marriage took place with the King's approval, the act itself was performed secretly and without consulting the great barons, as a marriage of such importance warranted.
Eleanor had previously been married to William Marshal, 2nd Earl of Pembroke, and she swore a vow of perpetual chastity upon his death, when she was sixteen, which she broke by marrying Montfort.
The Archbishop of Canterbury, Edmund Rich, condemned the marriage for this reason.
The English nobles protested the marriage of the King's sister to a foreigner of modest rank.
Most notably, the King's and Eleanor's brother Richard, 1st Earl of Cornwall, rose up in revolt when he learned of the marriage.
King Henry eventually bought off Richard with 6,000 marks and peace was restored.

The marriage brought the manor of Sutton Valence in Kent into Montfort's possession.
Relations between King Henry and Montfort were cordial at first.
Henry lent him his support when Montfort embarked for Rome in March 1238 to seek papal approval for his marriage.
When Simon and Eleanor's first son was born in November 1238 (despite rumours, more than nine months after the wedding), he was baptised Henry in honour of his royal uncle.
In February 1239, Montfort was finally invested with the Earldom of Leicester.
He also acted as the king's counsellor and was one of the nine godfathers of Henry's eldest son, Prince Edward, who would inherit the throne and become Edward I ("Longshanks").
In January 1238, Montfort married Eleanor of England, daughter of King John and Isabella of Angoulême and sister of King Henry III. While this marriage took place with the King's approval, the act itself was performed secretly and without consulting the great barons, as a marriage of such importance warranted. Eleanor had previously been married to William Marshal, 2nd Earl of Pembroke, and she swore a vow of perpetual chastity upon his death, when she was sixteen, which she broke by marrying Montfort. The Archbishop of Canterbury, Edmund Rich, condemned the marriage for this reason. The English nobles protested the marriage of the King's sister to a foreigner of modest rank. Most notably, the King's and Eleanor's brother Richard, 1st Earl of Cornwall, rose up in revolt when he learned of the marriage. King Henry eventually bought off Richard with 6,000 marks and peace was restored.
The marriage brought the manor of Sutton Valence in Kent into Montfort's possession. Relations between King Henry and Montfort were cordial at first. Henry lent him his support when Montfort embarked for Rome in March 1238 to seek papal approval for his marriage. When Simon and Eleanor's first son was born in November 1238 (despite rumours, more than nine months after the wedding), he was baptised Henry in honour of his royal uncle. In February 1239, Montfort was finally invested with the Earldom of Leicester. He also acted as the king's counsellor and was one of the nine godfathers of Henry's eldest son, Prince Edward, who would inherit the throne and become Edward I ("Longshanks").
Simon de Montfort, 6th Earl of Leicester