According to the 11th century hagiography, Vita Sancti Rumwoldi, he was the grandson of Penda of Mercia (a pagan, but incorrectly described in Rumwold's hagiography as having converted to Christianity) and the son of a king of Northumbria. His parents are not actually named; Alhfrith, son of Oswiu of Northumbria, did marry a daughter of Penda, Cyneburh, but Alhfrith was never king of Northumbria himself, although his father was (Alhfrith did rule the subkingdom of Deira for a time). There have, however, been doubts about whether these were his parents: for instance, the Northumbrian king is described as a pagan, but Alhfrith was a Christian (at least according to Bede, who says Alhfrith convinced Penda's son Peada to convert to Christianity). Although it has been stated that Cyneburh is not known to have had any children Northumbrian genealogy states she and Alhfrith had a further son Osric who subsequently became King of Northumbria himself (source: Stenton). In the Vita, Rumwold's mother is described as a pious Christian who, when married to a pagan king, tells him that she will not have sex with him until he converts to Christianity; he does so, and she becomes pregnant. The two are called by Penda to come to him when the time of her birth is near, but she gives birth during the journey, and immediately after being born the infant is said to have cried out: "Christianus sum, christianus sum, christianus sum" ("I am a Christian, I am a Christian, I am a Christian"). He went on to further profess his faith, to request baptism, and to ask to be named "Rumwold", afterwards giving a sermon. He predicted his own death, and said where he wanted his body to be laid to rest, in Buckingham.
Saint Rumwold is reported to have been born in Walton Grounds near King's Sutton in Northamptonshire which was at that time part of the Mercian royal estates possessing a court house and other instruments of government. The field in which he was born where a chapel once stood on the supposed spot may still be seen. Kings' Sutton parish church claims that its Saxon or Norman font may well have been the one Saint Rumwold was baptised in. There are two wells associated with his name in Astrop just outside Kings' Sutton and at Brackley and Buckingham where his relics once lay. Church dedications largely follow the missionary activity of Saint Wilfrid who was the personal chaplain of King Alhfrith (source: Bede) but once spread as far as North Yorkshire, Lincoln, Essex and Dorset.
In 2000 a complete Orthodox Christian service to Saint Rumwold was written along with a tone system (Orthodox musical system) with which to sing it which also has more general application. The service is performed on his two feast days which are November 3rd (main feast) and August 28th (translation of relics). In 2005 the former church of Saint Rumwold in Lincoln which is now a college erected a plaque to celebrate the connection.