The names of his parents and his ancestry are unknown. In 786, Cynewulf, king of Wessex, was killed by Cyneheard, brother of the former King Sigeberht. Either with help of Offa of Mercia or quickly coming under his influence, Beorhtric became King of Wessex. In 787, he held the Synod of Chelsea jointly with Offa. In 789 he married Eadburh, Offa's daughter. That same year Vikings appeared off the coast of Devon. Believing they were traders the local went to meet them and was killed. This was the first encounter with Vikings in southern England.