Harold acted as an emissary from Edward the Confessor to the court of William of Normandy in 1064, during which time he allegedly swore an oath of fealty to William, relinquishing any personal claim to the throne. This oath, which may have been given lightly, or possibly under duress, would figure directly in William's own claim, two years later. He would claim that the promise Harold made to him had been broken, giving William the right to challenge Harold in the battle for the crown.

While on his deathbed, the Confessor named Harold as his successor, overlooking his grandson, the rightful heir, Edgar the Ætheling, and ignoring a promise that he allegedly made (according to French sources) to William of Normandy. Upon Edward's death, Harold wasted no time securing ecclesiastical blessing on his claim by having himself crowned immediately.

Harold's brother, Tostig, had been exiled since the autumn of 1065 and had joined together with Harald Hardrada of Norway. A combined force landed in Yorkshire in September of 1066. Until this time, Harold's attention had been directed toward the south and the invasion that he knew would come from Normandy. But, now, Harold had to break away and march north to meet the new threat that had come. He defeated the forces of his traitorous brother and the King of Norway decisively at the battle of Stamford Bridge on the 25th of September.

Meanwhile, the favorable winds that the Normans had been waiting for had come and they had set sail across the channel, landing at Pevensey on the 28th. As soon as Harold heard this distressing news, he marched his force at top speed to the south. He reached London on October 5, and stopped to give his weary troops a rest and to gather reinforcements for the battle which lay ahead.

The story of these events and the decisive Battle of Hastings has been presented exquisitely in the Bayeux Tapestry and it need not be repeated, here. Suffice it to say that William won the day, and with it, the kingdom. The English fought fiercely and well, since they understood that not only their lives were at stake, but their country, also. Perhaps, if the English had been fresh and at full strength, they might have won easily, but they were tired and depleted after Stamford Bridge and the subsequent march south.

During his brief reign, the government continued to function as before, but there is no reliable way to judge what Harold might have been like as a king. He was certainly a capable field commander and a leader who inspired loyalty and confidence. His death has been recorded as coming in the midst of the final battle by way of a Norman arrow that penetrated his eye. Whether or not that is true, his memory lingers on as the last of the Anglo-Saxon kings, and the last monarch of England to suffer defeat at the hands of a foreign invader.