Ethelfleda married the lord of Mercia when she was 15 years old. The marriage was both a reward for Mercia's aid to King Alfred in driving the Danes out of Wessex, and a way of diplomatically maintaining Wessex's dominance over Mercia. Alfred awarded his son-in-law the title Earl of Mercia.
After the Earl's death, Ethelfleda ruled Mercia for seven years. When she died, Mercia became part of the Kingdom of Wessex.