[a] It was founded in the 6th century and covered the territory later occupied by the counties of Essex, Middlesex, much of Hertfordshire and (for a short while) west Kent.

The modern English county of Essex maintains the historic northern and the southern borders, but only covers the territory east of the River Lea, the other parts being lost to neighbouring Mercia during the 8th century.

[8] Saxon occupation of land that was to form the kingdom had begun by the early 5th century at Mucking and other locations.

[9] According to British legend (see Historia Brittonum) the territory known later as Essex was ceded by the Celtic Britons to the Saxons following the infamous Treason of the Long Knives, which occurred c. 460 during the reign of High King Vortigern.

[10] Studies suggest a pattern of typically peaceful co-existence, with the structure of the Romano-British landscape being maintained, and with the Saxon settlers believed to have been in the minority.

The historical identification of the kings of Essex, including the evidence and a reconstructed genealogy are discussed extensively by Yorke.

[20] Christianity is thought to have flourished among the Trinovantes in the 4th century AD (late Roman period); indications include the remains of a probable church at Colchester,[21] dating from some time after 320, shortly after the emperor Constantine the Great granted freedom of worship to Christians in 313.

[22] It is not clear to what extent, if any, Christianity persisted by the time of the pagan East Saxon kings in the sixth century.

The earliest English record of the kingdom dates to Bede's Historia ecclesiastica gentis Anglorum, which noted the arrival of Bishop (later Saint) Mellitus in London in 604.

A plague the same year caused Sigehere and his people to recant their Christianity and Essex reverted to Paganism a third time.

The inscription on the chest was recorded by Paul Hentzner and translated by Robert Naunton as reading: "Here lies Seba, King of the East Saxons, who was converted to the faith by St. Erkenwald, Bishop of London, A.D.

[32] However, by the mid-8th century, much of the kingdom, including London, had fallen to Mercia and the rump of Essex, roughly the modern county, was now subordinate to the same.

[36] Sometime between 878 and 886, the territory was formally ceded by Wessex to the Danelaw kingdom of East Anglia, under the Treaty of Alfred and Guthrum.