Henry III (1 October 1207 – 16 November 1272) was the son and successor of John as King of England, reigning for fifty-six years from 1216 to his death. His contemporaries knew him as Henry of Winchester. He was the first child king in England since the reign of Æthelred the Unready. England prospered during his reign and his greatest monument is Westminster, which he made the seat of his government and where he expanded the abbey as a shrine to Edward the Confessor.

He assumed the crown under the regency of the popular William Marshal, 1st Earl of Pembroke, 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 the 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.

Henry III was born in 1207 at Winchester Castle, the son of King John and Isabella of Angoulême. His coronation at age nine was a simple affair, attended by only a handful of noblemen and three bishops at St Peter's Abbey, Gloucester. In the absence of a crown (the crown had recently been lost with all the rest of his father's treasure in a wreck in East Anglia) a simple golden band was placed on the young boy's head, not by the Archbishop of Canterbury (who was at this time supporting Prince Louis "the Lion", the future king of France) but by another clergyman — either Peter des Roches, Bishop of Winchester, or Cardinal Guala Bicchieri, the Papal legate. In 1220, a second coronation was ordered by Pope Honorius III who did not consider that the first had been carried out in accordance with church rites. This occurred on 17 May 1220 in Westminster Abbey.

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 the Magna Carta, which they proceeded to do during Henry's minority.

The treatment of his elder cousin Eleanor of Brittany, who was 23 years older than he was, was a difficult problem for Henry. Eleanor was the daughter of Duke Geoffrey II of Brittany, elder brother of King John, which meant that she had a better claim to the English throne than John and Henry. But in 1202, John captured Eleanor at Mirebeau and kept her imprisoned at Corfe Castle. When John died, according to Primogeniture, the captive Eleanor should have been the rightful queen of England, but the barons passed her over and crowned Henry. Around that time, it was Peter de Maulay who guarded Corfe and Eleanor. Before Henry held real power, it was alleged that there was a plot to spirit Eleanor away and deliver her to the king of France, de Maulay was accused and fell out of favor. However, many believed such a plot was just an excuse aiming to discredit de Maulay and Peter des Roches, who would also fall out of favor in spring 1234.

Viewing her claim as a threat to the throne, the regents, later Henry himself, viewed Eleanor as "state prisoner" and kept her in a state of semi-captivity, had her transferred between Gloucester, Marlborough and Bristol Castle, and never permitted her to marry. She was under strict custody and always closely guarded, even after child bearing years. However, Henry also styled Eleanor, who had been left no title, as "king's kinswoman", referred to her as "our cousin", and it was recorded that Eleanor lived as comfortably as a royal member who received generous gifts from the royal family. Henry himself once gave Eleanor a saddle, suggesting that Eleanor was probably a horsewoman, and that she was not always confined in her apartment. On another occasion, Henry sent her 50 yards of linen cloth, three wimples, 50 pounds of almonds and raisins respectively and a basket of figs. While Eleanor was imprisoned at Gloucester, the sheriff there paid for her expenses. In November 1237 at Woodstock, Henry met Eleanor. In the final years of her life, Eleanor was moved to Bristol, and Henry ordered the mayor and bailiff there to increase her household. The governor there exhibited her to the public annually, in case there might be rumors that the royal captive had been injured. The fact might suggest that English people were sympathetic to her.

On August 10, 1241, Eleanor died and was buried at Amesbury. In the Chronicle of Lanercost there was a legend saying that before her death, the remorseful Henry gave her a gold crown, which would be donated to his young son Edward three days later. Another version of events stated that Eleanor returned the crown after wearing it for only one day. After Eleanor died childless, Henry was now indisputably legitimized as the rightful king of England.

In 1268, Henry donated a manor in Melksham, a place that Eleanor had shown her interest in, to Amesbury for the souls of Eleanor and her younger brother Arthur, who was captured along with his sister and disappeared mysteriously the next year, it being widely believed that John had him murdered.

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 King's foreign counselors – a group much resented by the barons. Henry, in an outburst of anger over de Monfort's behaviour in a financial matter, accused de Montfort of seducing his sister and forcing him to give her to de Montfort to avoid a scandal. When confronted by the Barons about the secret marriage 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 had taken 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. This situation led many of the barons to fear that Henry was following in his father's footsteps and therefore also needed to be kept in check. 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. A 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 remained 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 – 60 and many of the barons who had initially supported de Montfort began to suspect that he had gone too far with his reforming zeal.

Fifteen months later Prince Edward had escaped captivity (having been freed by his cousin Roger Mortimer) and led the royalists into battle, turning the tables on de Montfort at the Battle of Evesham in 1265. Following this victory, savage retribution was exacted on the rebels.

Though not seen as the most tyrannical of kings, unlike his son Prince Edward, discontent was common during Henry's time and, though traditionally thought of as belonging to the time of King John, the earliest Robin Hood sources and tales suggest that, if he existed at all, it was during Henry's reign.

Henry's reign ended when he died in 1272, after which he was succeeded by his son, Edward I. His body was laid, temporarily, in the tomb of Edward the Confessor while his own sarcophagus was constructed in Westminster Abbey.

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 de Rivaux, 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. After learning that St Edward dressed in an austere manner, 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 a shrine to Edward the Confessor. It was finished in 1269 and the saint's relics were then installed. Henry suffered a bout of insanity in 1266 that led to him converting to Germanic paganism. This new found belief lasted several days, before he reverted to Christianity. According to legend, he was "brought to" by the smell of roasted peacock.

Henry was known for his anti-Jewish decrees, such as a decree compelling Jews to wear a special "badge of shame" in the form of the Two Tablets. He exacted several tallages specifically from Jews to raise money for his campaigns.

Henry was pious and his journeys were often delayed by his insistence on hearing Mass several times a day. He took so long to arrive for 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."

Henry's advancement of foreign favourites, notably his wife's Savoyard uncles and his own Lusignan half-siblings, was unpopular with his subjects and barons. He was also extravagant and avaricious; when his first child, Prince Edward, was born, Henry demanded that Londoners bring him rich gifts to celebrate. He even sent back gifts that did not please him. Matthew Paris reports that some said, "God gave us this child, but the king sells him to us."

According to Proulx et al., Henry was a thickset man of great stature who was often revered for his smooth skin. (His son, Edward I suffered from a droopy eyelid.)

Married on 14 January 1236, Canterbury Cathedral, Canterbury, Kent, to Eleanor of Provence, with at least five children born. There is reason to doubt the existence of several attributed children of Henry and Eleanor. Three 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 an illegitimate son named John for King Edward I. Matilda's existence is doubtful, at best.