Queen of the Mercians
The date of Offa and Cynethryth's marriage is not known, but it was not until after the birth of Ecgfrith that Cynethryth began to witness charters. She first witnessed a charter dated 770, along with Ecgfrith and Ælfflæd. By 780 she is Cyneðryð Dei gratia regina Merciorum ("Cynethryth, by the Grace of God, Queen of the Mercians").
It has been suggested that Cynethryth's coinage was in emulation of the Byzantine Empress Irene, who ruled during this time through her son Constantine VI. The imagery employed, however, does not follow that on Irene's coinage, but that used on coins of late Roman empresses, just as the image used on Offa's coins show him as a late Roman emperor. It has been suggested that the coins were minted for donations by Cynethryth to the Church, but their similarity to the general issues suggests otherwise. This coinage is unique in Anglo-Saxon England, and indeed in Western Europe in this period.
Cynethryth is associated with her husband in charters and is said to have been a patron of Chertsey Abbey. Pope Adrian I, when elevating Higbert's Bishopric of Lichfield to an Archbishopric, wrote to Offa and Cynethryth jointly.