The only contemporary information about the person is provided by Gildas, who includes Maelgwn among the five British kings whom he condemns in allegorical terms in his "De Excidio et Conquestu Britanniae". He says Maelgwn held a regional pre-eminence among the other four kings, going on to say that he overthrew his paternal uncle () to gain the throne; that he had taken up life as a monk but then returned to the secular world; that he had been married and divorced, then remarried to the widow of his nephew after being responsible for his nephew's death; and that he was tall.