Ælfflæd was a daughter of Offa of Mercia

Offa of Mercia

Offa was the King of Mercia from 757 until his death in July 796. The son of Thingfrith and a descendant of Eowa, Offa came to the throne after a period of civil war following the assassination of Æthelbald after defeating the other claimant Beornred. In the early years of Offa's reign it is likely...

 and Cynethryth

Cynethryth

Cynethryth was the wife of Offa of Mercia and mother of Ecgfrith of Mercia. Cynethryth is the only Anglo-Saxon Queen consort in whose name coinage was definitely issued.-Origins and marriage:...

.

She may have witnessed a charter

Anglo-Saxon Charters

Anglo-Saxon charters are documents from the early medieval period in Britain which typically make a grant of land or record a privilege. The earliest surviving charters were drawn up in the 670s; the oldest surviving charters granted land to the Church, but from the eighth century surviving...

 with her father, mother, and brother Ecgfrith

Ecgfrith of Mercia

Ecgfrith was a King of Mercia who briefly ruled in the year 796. He was the son and heir of King Offa of Mercia and his wife Cynethryth. In 787, Offa had Ecgfrith crowned as co-ruler. He succeeded his father in July 796, but despite Offa's efforts to secure his son's succession, it is recorded...

 in the 770s, and again in 787 with her mother, father, brother, and two sisters; here she is described as virgo—unmarried.

It is possible that she was the daughter of Offa whose proposed marriage to Charles the Younger caused a dispute between Charlemagne

Charlemagne

Charlemagne was King of the Franks from 768 and Emperor of the Romans from 800 to his death in 814. He expanded the Frankish kingdom into an empire that incorporated much of Western and Central Europe. During his reign, he conquered Italy and was crowned by Pope Leo III on 25 December 800...

 and Offa in around 789–790.

In 792 she married Æthelred I of Northumbria

Æthelred I of Northumbria

Æthelred was king of Northumbria from 774 to 779 and again from 788 or 789 until his murder in 796. He became king after Alhred was deposed...

 at Catterick

Catterick, North Yorkshire

Catterick , sometimes Catterick Village, to distinguish it from the nearby Catterick Garrison, is a village and civil parish in the Richmondshire district of North Yorkshire, England...

. Here she is described as "queen", which has suggested to some historians that she had been previously married, and to a king, perhaps to one of Æthelred's predecessors.