While travelling to Mercia for her wedding her band was attacked by the Danes in an attempt to kill her and so sabotage the alliance between Wessex and Mercia. Though half her company perished in the first attack, Ethelfleda used an old trench as a fortress, and defeated the Danes. She was 15 when she married Aethelred or Ethelred, later the ealdorman or earl of Mercia, in about 886, and had one daughter, Ælfwynn.
During a sustained campaign of repeated attack between 865 and 878 the Danish Vikings overran most of the English Kingdoms such as Northumbria, Eastern Mercia, East Anglia and even threatened the very existence of Wessex. Alfred and his descendants reconquered these lands from the Danes by 937. The aid given him in this by Mercia had to be acknowledged. Instead of making the dominion of Wessex over Mercia seem like a conquest, Alfred married Ethelfleda to Aethelred of Mercia and gave his son-in-law the title Ealdorman or Earl of Mercia, thus allowing some ongoing autonomy. Since much of Western Mercia was never under the control of the Danes, and remained strong, this was a prudent move. Further prudence prevailed when the kingdoms were finally absorbed; they were not absorbed into Wessex or greater Wessex but into England. The term Anglo-Saxon thus reflects King Alfred's diplomatic integration of the Mercians Angles and the Saxons.
While her husband was alive, she signed agreements, leading some to think that she was the real leader. On her husband's death in 911 after the Battle of Tettenhall, she was elevated to the status of "Lady of the Mercians". This title was not a nominal position; she was a formidable military leader and tactician. Ethelfleda ruled for approximately eight years (according to the Anglo-Saxon Chronicle) from the newly fortified capital at Stafford, it is likely that the English county of Staffordshire first came into being during her reign. She fortified her existing borders and re-took Derby. She died at Tamworth in 918, and was buried at St Peter's Church (now St Oswald's priory) in Gloucester, which city (Gloucester) she had reconstructed from Roman ruins, and laid out the core street plan, which is still in existence today. She was joint lady of the Mercians along with her young daughter Aelfwynn.
The dominion of Mercia descended to Aelfwynn, Ethelfleda's heiress. Chroniclers have noticed the right of Aelfwynn so precisely as to leave no doubt concerning her claim; and this fact is of considerable value in showing that, contrary to the practice of other Germanic peoples, the sovereign authority amongst the Anglo-Saxons might descend to a female; or, according to the Anglo-Saxon expression, which the French have adopted, "fall to the spindle side".
However, Aelfwynn was compelled to submit to her mother's brother, King Edward the Elder of Wessex. The succession of Edward the Elder finalised the union of the two formerly separate kingdoms of Wessex and Mercia and gives some insight into the emergence of a unified England. In this instance, however, the weaker heir was compelled to yield to a more powerful opponent, and one from whom no enmity could have been feared. Aelfwynn was conducted as a captive into Mercia by her uncle Edward, who was engaged in successful warfare against the Danes; and we do not hear anything more concerning her in history. She seems to have lived the rest of her life in a nunnery.