Wulfhere, with an effective hegemony over all the Anglo-Saxon kingdoms except for Northumbria, had by the early 670s become the most powerful king in southern Britain.

Wulfhere according to Stephen of Ripon, had in 674 "stirred up all the southern nations against [Northumbria]", but he was defeated by Oswiu's son Ecgfrith who had forced him to surrender Lindsey, and to pay tribute.

Wulfhere survived the defeat but dies in 675, possibly of disease; his brother Æthelred succeeds him as king of Mercia.

The first recorded act of Æthelred's reign is in 676, when his armies ravage Kent, destroying Rochester, the seat of the bishops of West Kent.

The reason for his attack is not recorded, but he may have wished to prevent King Hlothhere of Kent from regaining control of Surrey, which had been recently brought into the Mercian orbit by Wulfhere.

It may also be that Æthelred wished for revenge for the murder of the sons of Eormenred of Kent; the murders had been instigated by Ecgberht of Kent, Hlothhere's brother, and it is possible that Æthelred was the uncle of the murdered princes.

A third suggestion is that the kings of Essex solicited the invasion, in response to recent Kentish attempts to gain dominance over the East Saxons.

Regardless of the reason, Hlothhere was likely then forced to accept Æthelred's overlordship.

The damage to the see of Rochester is so great that the incumbent bishop, Putta, retires from his diocese; his appointed successor, Cwichelm, also gives up the see "because of its poverty".

Æthelred is the son of Penda of Mercia.

Penda's queen, Cynewise, is named by Bede, who does not mention her children; no other wives of Penda are known and so it is likely but not certain that she was Æthelred's mother.

The Anglo-Saxon Chronicle gives Penda's age as fifty in 626, and credits him with a thirty-year reign, but this would put Penda at eighty years old at the time of his death, which is generally thought unlikely as two of his sons (Wulfhere and Æthelred) were young when he was killed.

At least as likely is that Penda was fifty years old at his death, rather than at his accession.

Æthelred's date of birth is unknown, but Bede describes Wulfhere as a youth at the time of his accession in 658, so it is likely he and Æthelred were in their middle teens at that time.

The early sources do not say whether Æthelred was older or younger than Wulfhere.

Nothing is known of Æthelred's childhood.

He had another brother, Peada, and two sisters, Cyneburh and Cyneswith; it is also possible that Merewalh, king of the Magonsæte, was Æthelred's brother.

Wulfhere in 674 "stirred up all the southern nations against [Northumbria]" according to Stephen of Ripon, but he was defeated by Oswiu's son Ecgfrith who forced him to surrender Lindsey, and to pay tribute.

Wulfhere survived the defeat, but died in 675, possibly of disease, and Æthelred has become king.

Theodore, the Archbishop of Canterbury, had begun a substantial reorganization of the church in Mercia early in the reign of Æthelred.

He had removed Wilfred from his position as Bishop of Lichfield in 675, and over the past four years has divided the vast Mercian see into the five dioceses of Leicester, Lichfield, Worcester, Dorchester and Hereford.

Æthelred, a devout king, has made several gifts of land to the expanding church, including grants at Tetbury, Long Newton, and Somerford Keynes.

There is also a tradition that Æthelred was associated with the founding of Abingdon Abbey, in southern Oxfordshire.

The group of six Anglican kingdoms—Kent, Sussex, Wessex, East Anglia, Essex, Northumbria, and Mercia—is known to historians as the Heptarchy.

Mercia has been in conflict with Northumbria since at least 633, when Penda of Mercia defeated and killed Edwin of Northumbria at the Battle of Hatfield Chase.

However, there have been diplomatic marriages between the two kingdoms: Æthelred's sister Cyneburh had married Alhfrith, a son of Oswiu of Northumbria, and both Æthelred and his brother Peada had married daughters of Oswiu.

Cyneburh's marriage to Alhfrith had taken place in the early 650s, and Peada's marriage, to Ealhflæd, had followed shortly afterwards; Æthelred's marriage, to Osthryth, is of unknown date but must have occurred before 679, since Bede mentions it in describing the Battle of the Trent, which takes place in this year.

Bede does not mention the cause of the battle, simply saying that it occurred in the ninth year of Ecgfrith's reign.

He is more informative on the outcome.

Ælfwine, the young subking of Deira, was killed; Ælfwine was brother to Osthryth and Ecgfrith, and was well liked in both Mercia and Northumbria since Æthelred's marriage to Osthryth.

His death according to Bede threatened to cause further strife between the two kingdoms, but Theodore, the Archbishop of Canterbury, intervened.

Æthelred takes possession of Lindsey again after the battle; the change in control this time will be lasting, and Lindsey will remain part of Mercia until the Viking invasion of the ninth century remakes the map of England.

Wilfrid has spent the past few years in Selsey, where he has founded an episcopal see and converted the pagan inhabitants of the Kingdom of Sussex to Christianity.

Theodore and Wilfrid have settled their differences, and Theodore has urged the new Northumbrian king, Aldfrith, to allow Wilfrid's return.

Aldfrith had agreed to do so, but in 691 he expels Wilfrid again.

Wilfrid goes to Mercia, where he helps missionaries and acts as bishop for the Mercian king.