He was the son of Offa, one of the most powerful kings of Mercia, and Cynethryth, his wife.

[1][2] Around 789, Offa seems to have intended that Ecgfrith marry the Frankish king Charlemagne's daughter Bertha, but Charlemagne was outraged by the request and the proposal never went forward.

[3] According to the Croyland Chronicle "he (Ecgfrith) was seized with a malady, and departed this life."

[4] Ecgfrith was succeeded by a distant relative, Coenwulf, presumably because Offa had arranged the murder of nearer relatives in order to eliminate dynastic rivals.

According to a contemporary letter from Alcuin of York, an English deacon and scholar who spent over a decade at Charlemagne's court as one of his chief advisors:[5] Alcuin added: "This was not a strengthening of the kingdom, but its ruin.