Aethelred was a younger son of King Edgar the Peacemaker and his second wife, Aelfthrith. He was only about nine years old when his father died in AD 975, but it was his elder half-brother, Edward, who was given the Crown. Queen Aelfthrith and her friend, St. Aethelwold, Bishop of Winchester, had opposed this succession and, when King Edward was assassinated three years later, the finger of suspicion naturally pointed in their direction. An old story tells how he was stabbed to death whilst Aelfthrith served him refreshments at Corfe Castle in Dorset. Aethelred was present at this palace throughout these events, but he is said to have known nothing of his mother's plot and was quite distraught upon hearing of his brother's murder. In reality, the assassination occurred because the anti-monastic Mercian faction in the country wanted rid of Edward. His death did not immediately resolve the power struggle though and Aethelred was not crowned until over a year later.

During his teenage years, Aethelred was guided by his mother, Bishop Aethelwold of Winchester and Ealdorman Aelfhere of Mercia. However, the last two died in AD 983 & 4, when new influences came into play, forcing the Dowager Queen to withdraw from public life. While the country suffered once more from Viking attacks, greedy counsellors persuaded the young King to reduce the privileges, and thus the power, of the reformed Church, particularly by confiscating its lands and redistributing them amongst his magnates. In AD 985, Aethelred married Aelfgitha, the daughter of Ealdorman Thored of Northumbria.

After the defeat of the English forces, by King Olaf Tryggvason, at the Battle of Maldon in AD 991, Aethelred took to paying the invaders tribute (£10,000) in order to persuade them to leave. Within two years, Aethelred had repented his anti-ecclesiastical actions and turned his back on his former associates. His mother returned and local government was reorganized, with the suppression of the hereditary earldordoms in favour of more dependent sheriffs and high-reeves. There was a corresponding resurgence of church reformation and, though Viking attacks grew worse (with further payments of £16 & £24,000), the Royal burghs were refurbished and the next decade became a much more stable period for England. It is not surprising, therefore, that a flowering of art and literature followed: notably through the works of Aelfric, Bertferth and Wulfstan of Winchester.

In 1002, Aethelred married for a second time. He chose Lady Emma, the sister of Duke Richard II of Normandy, as his bride because he wanted to draw the Duke away from his Viking allies, who were using Normandy as a base from which to attack England. The alliance was successful for, on St Brice's Day (3rd November) of the following year, Aethelred felt secure enough to order that "all the Danish people who were in England" were to be slain "because [he] had been told that they wished to deprive him of his life by treachery". The massacre was enacted upon Viking merchants and recent settlers rather than the many Anglo-Danish families of the Danelaw, but it still made the King unpopular amongst his northern subjects.

In 1006, there was another great reshuffle within the King's Council and his chief counsellor, Edric Streona, began his rise to infamy. The same year saw a major Viking invasion which was only halted by a massive payment £36,000. Aethelred decided to, thenceforward, put all his efforts into beating back his enemies before they even reached English shores. He commanded that "ships should be built unremittingly over all England, namely, a warship from every three hundred hides and a helmet and a corselet from every ten". Unfortunately, however, feuds involving Streona's family restricted their usefulness and, in 1009, yet another huge Viking army landed. Led by Thorkell the Tall, the Norsemen ravaged most of Southern England for the next three years. There most notable clash being against the great Ulfkell Snilling at the Battle of Ringmere in 1010. Only after sacking Canterbury and kidnapping, and then murdering, the Archbishop, St. Alphege, were they payed £48,000 to stop. At this point, the Vikings decided to swap sides and offered their mercenary services to King Aethelred, which he eagerly accepted in return for an annual payment of 'heregeld'.

Unfortunately, such measures did not prevent the Danish King, Swein Forkbeard, from landing at the mouth of the Humber in August 1013. The Danelaw immediately submitted to his rule, so Swein's vast army passed without incident through the North. Beyond Watling Street, they began to ravage the countryside to such an extent that East Anglia and Wessex soon followed the Northern example. King Aethelred & Thorkell were besieged in London. They managed to hold out while Swein turned his attention to Mercia; but, by Christmas, Aethelred was forced to flee to Normandy and "the whole nation accepted [Swein] as full king".

However, the new regime lasted but a month. Swein died unexpectedly on 2nd February and the English nobles rejected his son, Canute. They sent word to Aethelred, declaring that "no lord was dearer to them than their rightful lord, if only he would govern his kingdom more justly than he had done in the past". One of Aethelred's youngest sons, Edward, brought his reply: that he "would remedy each one of the things that they all abhorred," and the King made a triumphant return. Canute was driven from the country.

Dissension amongst the Royal family appears to have led to Aethelred's final downfall. His eldest surviving son, Edmund, seems to have been alarmed by the favour shown to his half-brother in the negotiations of 1014. When, the following year, Aethelred ordered the murder of the leading Northern magnates, Sigeferth and Morcar, Edmund made his own play for power in the Danelaw by marrying Sigeferth's widow. The region submitted to the prince's authority and Aethelred was enraged. Later the same year, Canute invaded England once more, but the English troops were divided: the South rallied to the King's right-hand-man, Edric Streona, but the North looked to Edmund's leadership. Only Aethelred could unite his armies, but in the battles of 1015 &16, he was already an ill man. Unfortunately, a reconciliation between father and son came too late to salvage the situation. Aethelred died on 23rd April 1016, was buried in St. Paul's Cathedral (London) and succeeded by his son, Edmund.

By his first wife, Aelfgitha, Aethelred had a very large family: Aethelstan, Egbert, Edmund, Edred, Edwig, Edward (who died young) and Edgar, as well as five daughters including Edith, Aelfgitha and Wulfhilda. By his second wife, Emma, there were three children, Edward, Alfred and Godgitha.

Aethelred's epithet, commonly said to be 'Unready,' dates from sometime after his reign. It has nothing to do with being ill-prepared, but means 'the Ill-Advised'; a name for which he was well suited. As the King himself admitted, in AD 993, his advisers were often able to take advantage of his own ignorance. While his support of the treacherous Edric Streona, after 1006, shows the King to have been an extremely poor judge of character. However, we should not be too hard on Aethelred. Being forced to deal with Viking invasions throughout a thirty-eight year reign, was not easy. Narratives, written during the difficult latter period of his reign, have tarnished his reputation and obscured his positive achievements.