Shortly after becoming king, he was driven out of Kent by an invasion of Offa of Mercia. He briefly returned to power 20 years later, following the death of Egbert II, but his second reign ended about as quickly as the first: Offa again invaded Kent, and Ealhmund was probably killed in battle. Except for brief periods of rebellion, this effectively marked the end of Kent's independence, as it was absorbed by more powerful Anglo-Saxon kingdoms: first by Mercia, and then by Wessex 40 years later.

There is little historical evidence for Ealhmund's reign. A charter of 784 mentions him, and the 784 entry of the Anglo-Saxon Chronicle mentions Eahlmund as the father of Egbert, but this is regarded as tenuous.