At the start of the reign of King Æthelstan in 924, Beornstan was a member of his household, one of his mass priests, who were probably responsible for looking after his relics.
Early in Æthelstan's reign, Beornstan witnessed his manumission of a slave called Ealdred.
Æthelstan followed a policy of appointing members of his own circle to vacant bishoprics in Wessex.
Winchester was a centre of opposition to the king under its bishop, Frithestan, and when he resigned in 931 Æthelstan took the chance to appoint Beornstan to the position.
He frequently attested the king's charters, though in a lower position than his successor, Ælfheah.