Cedd was soon recalled from the mission to Mercia by Oswiu himself. The king then sent him to the East Saxon kingdom, accompanied by one other priest. This was at the request of King Sigeberht to re-convert his people.

The East Saxon kingdom was originally converted by missionaries from Canterbury, where St. Augustine had established a Roman mission in 597. The first bishop of the Roman Rite was Mellitus, who arrived in Essex in 604, but he had been driven out after about a decade. Thereafter, the religious destiny of the kingdom was constantly in the balance, with the royal family itself divided - some Christian, some pagan, and some wanting to tolerate both.

Bede tells us that Sigeberht's decision to be baptized and to reconvert his kingdom definitively was on the initiative of Oswiu. In fact Sigeberht travelled to Northumbria to accept baptism from Bishop Finan of Lindisfarne. It seems that Cedd went to the East Saxons partly as an emissary of the Northumbrian monarchy. Certainly his prospects can only have been helped by the continuing success of Northumbria, especially the final defeat of Penda in 655.

After making some conversions, Cedd returned to Lindisfarne to report to Finan. In recognition of his success, Finan ordained him bishop, calling in two other Irish bishops to assist at the rite. Cedd was appointed bishop of the East Saxons.

Bede's record makes clear that Cedd demanded personal commitment and that he was unafraid to confront the powerful. He excommunicated a thegn who was in an unlawful marriage and forbade Christians to accept the man's hospitality. According to Bede, when Sigeberht himself continued to visit the man's home, Cedd descended on their revels to denounce the king openly, foretelling that he would die in that very house. Bede asserts that the King's subsequent murder (660) was his penance for defying Cedd's injunction.

There are signs that Cedd's position in Essex became more tenuous after the death of Sigeberht. The new king, and murderer of Sigeberht, Swithelm, was a pagan. It seems that he had long been a client of Ethelwald, king of the East Angles, who was himself increasingly dependent on Wulfhere, Christian king of a newly resurgent Mercia. After some persuasion from Ethelwald, Swithelm accepted baptism from Cedd, although Cedd had to travel into East Anglia to baptize him at Ethelwald's home. This kept the East Saxon kingdom Christian for the time being.

Bede presents Cedd's work as decisive in the conversion of the East Saxons. This is despite earlier missionary work and a subsequent relapse into paganism. It seems that substantial work had been done but that there was still a possibility of that it could be undone.