The start of Edward's reign was less momentous. He had been raised to the throne following the forced abdication of his father. He was only fourteen and was in the manipulative hands of his mother, Isabella of France, and her paramour, Roger Mortimer. They had detained Edward in France the previous year when he had gone to pay homage to the French king Charles IV for his lands in Aquitaine and Ponthieu, and it was under Isabella's standard that Edward returned to England in September 1326 to be proclaimed "Keeper of the Realm". Edward remained under the control of Isabella and Mortimer, though there was some saving grace in that the head of the regency council was Henry of Lancaster, a cousin of Edward's father and a more moderate man than most. The murder of Edward's father at the instigation of Mortimer shocked the nation, but the young king was in no position to do much about it at the time. Despite his youth he had led an army into northern England in the summer of 1327 to counter the attacks made by the Scots on the borders. After a month trying to track down the enemy amongst the wild country, Edward was forced to admit defeat and he returned dejected to York. Negotiations were opened with the Scots resulting in the Treaty of Northampton in May 1328 where the independence of the Scots was recognized. Edward's sister Joan was betrothed to Robert The Bruce's infant son, David (II), and they were married that July at Berwick. Edward in the meantime had married his long-time betrothed, Philippa of Hainault, who had travelled to York for the wedding in January 1328. Their marriage was happy and fruitful, far more so than England's alliance with Scotland which was always threatened by the uneasy menage a trois between Scotland, England and France. That same year Charles IV of France died without an heir. Edward believed he had a more direct claim, through his mother, as grandson of Philippe IV of France and nephew of Charles IV. The French did not recognize inheritance through the female line and overturned Edward's claim in favour of Charles's cousin, Philippe de Valois. In any case there was no way that the French would be ruled by an English king, but that did not stop Edward's ambitions. He did not press his claim for the moment, but paid homage to the new king for his lands in France, and bided his time.

In the meantime Edward took control of his own affairs. There was a rising tide of feeling against his mother, Isabella, and Mortimer, who were now living openly together. Mortimer was continuing to seek retribution against the supporters of Edward II, including the young king's uncle, Edmund, earl of Kent, who was executed in March 1330. Edward decided that enough was enough. With the support of Henry of Lancaster, Edward's men (including William de Montacute) seized Mortimer at Nottingham Castle, apparently dragging him from Isabella's embrace. He was tried and convicted, and was hanged, drawn and quartered at the new place of execution at Tyburn in London. He was the first of many thousands to be executed there over the next 450 years. On 20 October Edward assumed personal responsibility. He was to receive unanimous support from his barons. They recognized in him a quality that reminded them of the great days of Edward I. Edward used the facilities of the Parliament to ensure that the barons received a fair opportunity to contribute to debates, and he often took their advice. Whilst he remained superior, in the regal sense, he did not demean the barons.

Clearly none of this would have worked had Edward not proved himself an able soldier and king. His first opportunity came against the long standing enemy, Scotland. Edward Balliol maintained his right to the throne of Scotland against the family of Bruce and Edward supported his claim. In August 1332 Edward overthrew David II. Even though David was Edward's brother-in-law, Edward continued to support Balliol, and Balliol recognized him as his overlord at Roxburgh on 23 November 1332. Consequently, when Balliol was himself overthrown a month later, Edward III responded, providing forces to support Balliol's endeavours to regain the throne. Edward's army won a crushing victory over the Scots at Halidon Hill on 19 July 1333. Here, for the first time, the strength of the English and Welsh longbowmen played their part in seriously weakening the Scots forces. Balliol was restored and a year later, in June 1334, gave to the English crown almost all of the border country between the Forth and the Tweed, which was immediately governed as part of England. The Scots however fought back. Balliol was deposed again, and although he soon regained the throne it became evident that Balliol could only rule with English support. When he was overthrown again in 1336, he gave up and David II was restored. Edward did not pursue rights to Scottish territory, because his attention shifted to France. France and Scotland had long been allies and Edward was only too aware that united the two could seriously damage England. Moreover the many Gascons living in England had a right of appeal in the courts to the French king, through Edward's fealty for his lands in Gascony. Edward recognized that these problems would be solved if his claim to the throne of France was upheld. He revived this claim in 1337 and declared his intentions to fight for it. This was the start of what became known as the Hundred Years' War. Supported by his barons, Edward crossed to Antwerp and invaded France in 1338. Philippe VI refused to fight, and the next eighteen months saw only minor and relatively unsuccessful skirmishes. However, in 1340 Edward won a major sea battle off the port of Sluys in Holland, and this emboldened him enough to declare himself king of France. He even challenged Philippe to decide the matter in single combat, but Philippe refused. For the next three years Edward was unable to make any advance, and the cost of maintaining his army and fleet became crippling. He reached a truce with Philippe in 1343, but that truce was conveniently broken in 1345. Now Edward's luck changed. His great general, Henry, earl of Derby, regained Gascony, and Edward followed up with a major invasion force. Landing in Normandy in July 1346, he harried his way through northern France with much pillaging and destruction, which galvanised Philippe into action. The two armies met at Crecy, near the Somme, on 26 August. Again it was the power of the longbow that gave the victory to Edward. His army moved on to Calais, to which he laid siege for several months until the citizens submitted, on the verge of starvation. Edward was prepared to sack and destroy the city but his queen, Philippa, who had accompanied him on the campaign, pleaded for their lives. While the siege was continuing Edward received excellent news from England. The Scots had sought to take advantage of Edward's absence by invading northern England, but their forces were routed by the archbishop of York at Neville's Cross in October 1346. David II was captured and taken as prisoner to England. A peace treaty was concluded in France and Edward returned to England.

The next few years may be seen both as the zenith of Edward's reign but also some of the darkest days in Britain. Edward, triumphant in his victory over France, and with David of Scotland his prisoner, established a court par excellence at Windsor Castle. Edward operated his Court on the model of the Arthurian Round Table. Arthur was his hero, and many of the incidents later related by Thomas Malory in his Morte d’arthur have their counterparts in Edward's tournaments and chivalric quests. The world of Edward III was the world of Arthur. Edward planned to instigate an Order of the Round Table, which was eventually called the Most Noble Order of the Garter when he established it in 1348. It was the highest order of chivalry limited always to a select group of twenty-five or so knights. It was first bestowed upon Edward's eldest son, Edward, the Black Prince, and included among its illustrious ranks his second cousin, Henry, earl of Derby (later duke of Lancaster) (and grandfather of the future king Henry IV), and Roger Mortimer, the grandson of his mother's lover. These honours and the opportunity to prove themselves to the king resulted in a rare camaraderie between the king and his nobles, one which helped sustain the successes of the first half of Edward's reign. Edward called to his court the greatest knights from throughout Europe who would prove their valour and strength in his tournaments. It was a period of considerable glory and prestige for England. However it was darkened by the Black Death, the name given to the virulent bubonic plague that swept through Europe, reaching its height in the north in 1348, and which was believed to have killed a third of the population of Britain (or almost a million people). Over two hundred people a day died in London alone for over two years. Edward's thirteen-year old daughter Joan died of it in Bordeaux in September 1348, as did his infant son Thomas, but although others of the court succumbed, the plague left the royal family surprisingly unscathed. Nevertheless it devastated the English economy and reduced the manpower on which Edward could call, but this was the same throughout Europe. It was through the remarkable ability of Edward's officials that the finances and administration of England were sustained through this period.

Philippe VI of France died in August 1350 and with the succession of his son, Jean II, hostilities broke out again with England. Jean, known as the Good, refused to acknowledge Edward's overlordship but likewise refused to fight until Edward's tactics of plunder and destruction drew the French out. A series of battles, of which the most decisive was at Poitiers in September 1356 under the command of the Black Prince, brought French government to the verge of anarchy. Jean II was taken captive to England. Unable to agree terms, Edward III invaded France in 1359, hoping at last to gain control. However, the continuance of the bubonic plague and a devastatingly severe winter weakened the English as much as the French and Edward was unable to strike the final blow. A treaty was nevertheless agreed at Bretigny in 1360. By this Edward's sovereignty was recognized over his former lands in France, primarily Aquitaine and Calais. In return Edward dropped his claim to the throne of France.

The year 1360 remains the peak of Edward's reign as the remaining sixteen years were ones of slow and sad decline. The plague returned with increasing virulence in 1361. The French king Jean was never able to raise his ransom (set at half a million pounds) and died in London in 1364. His son, Charles V, ascended the French throne and encouraged the French subjects of the English in Aquitaine to rebel. Although Edward tried to negotiate with France relations worsened. The great triumphs of the Black Prince faded as his health failed, and were replaced by the ignominious defeats of his brother John of Gaunt. The war became protracted and costly and, when the French plundered and burnt Portsmouth in 1369, English opinion began to turn away from the king. This was further aggravated when Edward drew upon church revenues in order to finance the conflict. By 1374 Edward had lost much of Aquitaine, and the French regained control over most of the English territories except Calais and a coastral strip in southern Gascony, near Bayonne.

Edward's health and spirit also failed. His wife, Philippa, died in August 1369 of an illness akin to dropsy. They had been married for forty years and by all accounts had remained very happy. Philippa had grown rather portly in her last years which added to her overall character as a friendly, homely, motherly woman whom the nation greatly loved. In her final years Edward had found himself drawn to Alice Perrers, one of his wife's chamber ladies, who became his mistress. Although she satisfied the old king's physical desires - in fact she bore him at least three children - she was also scheming and grasping and made his final years a misery. She was banished from the court in 1376. It was likely that she was the cause of the king contracting gonorrhoea. To add to the king's sadness, his eldest son, Edward the Black Prince, who had turned against his father's policies, died in June 1376. The king's final year was spent in much loneliness and sadness aware that the administration about him was crumbling. He died of a stroke at Sheen Palace in June 1377. He was succeeded by his grandson, Richard II. Although his reign is remembered for its great days of glorious knighthood and conquest, it was this and the Black Death that drove the country into poverty and near ruin and was the basis for the collapse of the Plantagenet dynasty in the next generation. Edward III cannot be denied the epitaph of a great king, but like so many other great kings he bequeathed little to his successors.