Nothing certain is known of Cynethryth's origins. Her name recalls the wife and daughters of King Penda—Cynewise, Cyneburh, and Cyneswith—which may indicate that she was a descendant of Penda. A tradition related by the 13th centuryVitae duorum Offarum tells that she was of Frankish origin, and that for her crimes she was condemned by Charlemagne's justice system to be set adrift at sea in an open boat. The boat eventually stranded on the Welsh coast where she was taken to Offa. She pleaded that she had been cruelly persecuted and was of the Carolingianroyal house. Offa left Drida, as she was called, in the charge of Marcellina, his mother. Offa would fall in love with and marry her, at which point she adopted the name Quindrida, but she continued in her iniquitous ways before being murdered by robbers. This seems to relate to a brief mention of Offa's sinful but reformed wife, Thritha, that appears in Beowulf, but also has aspects similar to a story told of the wife of Offa of Angel, a Yorkshire girl set adrift by her father. Unlike the relations of Æthelbald, Offa's predecessor, which had been condemned by the church, the marriage of Offa and Cynethryth was entirely conventional and met with the approval of the church hierarchy. In a letter to Cynethryth and Offa's son Ecgfrith, Alcuin advises him to follow the example of his parents, including his mother's piety. Elsewhere Alcuin refers to Cynethryth as "controller of the Royal household".
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. 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.
Æthelberht II, King of East Anglia – who was later canonised as Saint Ethelbert the King – is widely believed to have been assassinated, on Offa's orders. Some later chroniclers, such as Roger of Wendover, have alleged that Cynethryth was either personally responsible for the assassination of Æthelberht, or to have incited Offa to kill him.
