In around 655 Alhfrith was appointed by his father as sub-king of Deira, the southern part of the Northumbrian kingdom. He replaced his cousin Æthelwold, who had supported Oswiu's enemy Penda of Mercia in the campaign leading up to the Battle of the Winwaed. Alhfrith was married to Penda's daughter Cyneburh; Cyneburh's brother Peada was doubly Alhfrith's brother-in-law as he later married Alhfrith's sister Ealhflæd.
At the Synod of Whitby in 664, Alhfrith was the chief supporter of Wilfrid. Bede, in the Historia ecclesiastica gentis Anglorum (Book III, chapter 14), states that Alhfrith attacked his father. No further details are known. Bede's Lives of the Abbots states that Alhfrith asked his father for permission to accompany Benedict Biscop on a pilgrimage to Rome, but the dating of this request is unclear. With this, Alhfrith disappears from the record.
While generally presumed to be the son of Aldfrith, a half-brother of Alhfrith, the possibility is admitted that Osric may have been a son of Alhfrith and Cyneburh.