Æthelflæd was the eldest daughter of King Alfred the Great — born in Wessex around 870 to Alfred and his queen Ealhswith. She was educated alongside her brother Edward (the future King Edward the Elder) in the scholarly atmosphere of Alfred's court.
Around 886 she was married to Æthelred, Lord of the Mercians (the sub-king of the Mercian kingdom under Wessex overlordship) — a dynastic marriage uniting the two great Anglo-Saxon kingdoms in their shared resistance to the Viking Danes.
Their only child was a daughter, Ælfwynn (born c. 888), born after a difficult childbirth — after which (per William of Malmesbury's tradition) Æthelflæd chose celibate continence for the rest of her marriage, the characteristically Anglo-Saxon Catholic royal women's-spiritual decision.
When her husband Æthelred fell ill around 902 and died in 911, Æthelflæd assumed sole rule of Mercia as the unique title 'Lady of the Mercians' (Myrcna hlædige) — the only woman in Anglo-Saxon history to rule an Anglo-Saxon kingdom in her own name (her brother Edward the Elder ruled Wessex; together they conducted the coordinated Anglo-Saxon Catholic reconquest of the Danelaw from the Viking Danes).
Between 910 and her death in 918, Æthelflæd built ten major fortified burhs across Mercia (Bridgnorth, Tamworth, Stafford, Warwick, Chirbury, Eddisbury, Weardbyrig, Runcorn, Bremesburh, Sceargate) and led Mercian armies in the reconquest of Derby (917) and Leicester (918) from the Danes. By the summer of 918, the Vikings of York had submitted to her lordship — but she died at Tamworth on June 12, 918 before the submission could be formalized.
She and her husband Æthelred had founded around 900 the St. Oswald's Priory in Gloucester (the New Minster) — translating the relics of St. Oswald of Northumbria there from Bardney — and Æthelflæd was buried there alongside her husband. The medieval Catholic Mercian tradition venerated her as 'Sancta Ethelfleda' — the queen-warrior saint of the Mercian Catholic kingdom.