He assumed the crown under the regency of the popular William Marshal, but the England he inherited had undergone several drastic changes in the reign of his father. He spent much of his reign fighting the barons over Magna Carta and the royal rights, and was eventually forced to call the first "parliament" in 1264. He was also unsuccessful on the Continent, where he endeavoured to re-establish English control over Normandy, Anjou, and Aquitaine.
Under John's rule, the barons had supported an invasion by Prince Louis because they disliked the way that John had ruled the country. However, they quickly saw that the young prince was a safer option. Henry's regents immediately declared their intention to rule by Magna Carta, which they proceeded to do during Henry’s minority. Magna Carta was reissued in 1217 as a sign of goodwill to the barons and the country was ruled by regents until 1227.
As Henry reached maturity he was keen to restore royal authority, looking towards the autocratic model of the French monarchy. Henry married Eleanor of Provence and he promoted many of his French relatives to higher positions of power and wealth. For instance, one Poitevin, Peter des Riveaux, held the offices of Treasurer of the Household, Keeper of the King's Wardrobe, Lord Privy Seal, and the sheriffdoms of twenty-one English counties simultaneously. Henry's tendency to govern for long periods with no publicly-appointed ministers who could be held accountable for their actions and decisions did not make matters any easier. Many English barons came to see his method of governing as foreign.
Henry was much taken with the cult of the Anglo-Saxon saint king Edward the Confessor who had been canonised in 1161. Told that St Edward dressed austerely, Henry took to doing the same and wearing only the simplest of robes. He had a mural of the saint painted in his bedchamber for inspiration before and after sleep and even named his eldest son Edward. Henry designated Westminster, where St Edward had founded the abbey, as the fixed seat of power in England and Westminster Hall duly became the greatest ceremonial space of the kingdom, where the council of nobles also met. Henry appointed French architects from Rheims to renovate Westminster Abbey in the Gothic style. Work began, at great expense, in 1245. The centrepiece of Henry's renovated abbey was to be a shrine to Edward the Confessor. It was finished in 1269 and the saint's relics were then installed. Henry was known for his anti-Jewish decrees, such as a decree compelling them to wear a special "badge of shame" in the form of the Two Tablets. Henry was extremely pious and his journeys were often delayed by his insistence on hearing Mass several times a day. He took so long to arrive on a visit to the French court that his brother-in-law, King Louis IX of France, banned priests from Henry's route. On one occasion, as related by Roger of Wendover, when King Henry met with papal prelates, he said, "If (the prelates) knew how much I, in my reverence of God, am afraid of them and how unwilling I am to offend them, they would trample on me as on an old and worn-out shoe."
In 1244, when the Scots threatened to invade England, King Henry III visited York Castle and ordered it rebuilt in stone. The work commenced in 1245, and took some 20 to 25 years to complete. The builders crowned the existing moat with a stone keep, known as the King's Tower.
Henry's reign came to be marked by civil strife as the English barons, led by Simon de Montfort, demanded more say in the running of the kingdom. French-born de Montfort had originally been one of the foreign upstarts so loathed by many as Henry's foreign councillors; after Henry, on an anger outbrust, accused Simon of seducing his sister and forcing him to give her to Simon to avoid an scandal, when confronted by the Barons about the secret marriage between the two that Henry had allowed to happen, a feud developed between the two. Their relationship reached a crisis in the 1250s when de Montfort was brought up on spurious charges for actions he took as lieutenant of Gascony, the last remaining Plantagenet land across the English Channel. He was acquitted by the Peers of the realm, much to the King's displeasure.
Henry also became embroiled in funding a war in Sicily on behalf of the Pope in return for a title for his second son Edmund, a state of affairs that made many barons fearful that Henry was following in the footsteps of his father, King John, and needed to be kept in check, too. De Montfort became leader of those who wanted to reassert Magna Carta and force the king to surrender more power to the baronial council. In 1258, seven leading barons forced Henry to agree to the Provisions of Oxford, which effectively abolished the absolutist Anglo-Norman monarchy, giving power to a council of fifteen barons to deal with the business of government and providing for a thrice-yearly meeting of parliament to monitor their performance. Henry was forced to take part in the swearing of a collective oath to the Provisions of Oxford.
In the following years, those supporting de Montfort and those supporting the king grew more and more polarised. Henry obtained a papal bull in 1262 exempting him from his oath and both sides began to raise armies. The Royalists were led by Prince Edward, Henry's eldest son. Civil war, known as the Second Barons' War, followed.
The charismatic de Montfort and his forces had captured most of southeastern England by 1263, and at the Battle of Lewes on 14 May 1264, Henry was defeated and taken prisoner by de Montfort's army. While Henry was reduced to being a figurehead king, de Montfort broadened representation to include each county of England and many important towns—that is, to groups beyond the nobility. Henry and Edward continued under house arrest. The short period that followed was the closest England was to come to complete abolition of the monarchy until the Commonwealth period of 1649–1660 and many of the barons who had initially supported de Montfort began to suspect that he had gone too far with his reforming zeal.
But only fifteen months later Prince Edward had escaped captivity (having been freed by his cousin Roger Mortimer) to lead the royalists into battle again and he turned the tables on de Montfort at the Battle of Evesham in 1265. Following this victory savage retribution was exacted on the rebels.
There is reason to doubt the existence of several attributed children of Henry and Eleanor.
Are known only from a 14th century addition made to a manuscript of Flores historiarum, and are nowhere contemporaneously recorded.
Another daughter, Matilda, is found only in the Hayles abbey chronicle, alongside such other fictitious children as a son named William for King John, and a bastard son named John for King Edward I. Matilda's existence is doubtful, at best. For further details, see Margaret Howell, The Children of King Henry III and Eleanor of Provence (1992).
Henry has been portrayed on screen as a child by Dora Senior in the silent short King John (1899), a version of John's death scene from Shakespeare's King John, and by Rusty Livingstone in the BBC Shakespeare The Life and Death of King John (1984). He was portrayed as an adult by Richard Bremmer in Just Visiting (2001), a remake of the French time travel film Les Visiteurs.