As with the other kings of East Anglia, there is very little textual information available. He did, however, leave an extensive coinage of both portrait and non-portrait type (for example, Coins of England and the United Kingdom, Spink and Son, London, 2005 and the Fitzwilliam Museum database of early medieval coins.
It is suggested that Æthelstan was probably the king who defeated and killed the Mercian kings Beornwulf (killed 826) and Ludeca (killed 827). He may have attempted to seize power in East Anglia on the death of Coenwulf of Mercia (died 821). If this is the case, he was apparently defeated by Coenwulf's successor Ceolwulf.
The end of Æthelstan's reign is placed in the middle or late 840s. He was succeeded by Æthelweard.