Celtic hagiography tells us that Brychan was born in Ireland, the son of a Prince Anlach, son of Coronac, and his wife, Marchel, heiress of the Welsh kingdom of Garthmadrun (Brycheiniog), which the couple later inherited. Upon his father's death, he returned to Garthmadrun and changed its name to Brycheiniog. Brychan's name may be a Welsh version of the Irish name Broccán and that of his grandfather Coronac may represent Cormac. The Life of St. Cadoc by Lifris (c. 1100) portrays Brychan fighting Arthur, Cai and Bedivere because of King Gwynllyw of Gwynllwg's abduction of his daughter St. Gwladys from his court in Talgarth.
He is occasionally described as an undocumented saint but the traditional literature does not call him a saint, referring to him as a patriarch instead, and he has no churches named for him. A 15th-century stained glass window in the parish church at St Neot in Cornwall, supposedly depicts Brychan, seated and crowned, holding in his arms eleven children. This, however, has been described by a standard modern guide as "God with Souls in his lap".
According to Christian tradition, Brychan was married three times – to Prawst ferch Tydwal, Banhadlwedd ferch Banadi, and Gwladys – and had a very large family. These wives are mentioned in several manuscripts, including those by William Worcester, John Leland, and Nicholas Roscarrock. The number of children attributed to him varies from twelve to sixty-three, the number most frequently encountered being twenty-four. There are two main lists however, one of Welsh origin and one of Cornish origin. Most of his children appear to have travelled from Brecon to evangelise Cornwall and North Devon, where they are now venerated, but there is little agreement between the two lists. Some are referred to as being "in Manau", which has led to associations of Brychan with Manaw Gododdin in modern Scotland; although the Isle of Man seems more likely.
The number of Brychan's children may have grown over time, as more and more secular people as well as saints wished to claim descent from one of the "Holy Families of Britain". Listed below are children from Welsh, Cornish, Irish, and Breton sources:
The sons listed in the Cognacio Brychan, De Situ Brecheniauc and the genealogies of Jesus College MS 20 are Cynog, Rhain Dremrudd, Clydwyn, Arthen, Papai, Dingad, Berwyn and Rhydog. Also listed, but not in all three, are Cynon, Pasgen, Cylflifer, Marthaerun and Rhun. Other Welsh sources claim the following additional sons: Caian, Cynbryd, Cynfran, Cynin, Dogfan, Dyfnan, Dyfrig, Hychan, Llecheu, Neffei, Rhawin, Llofan, Llonio, Heilin, Afallach, Gwynnen and Gwynnws.
The De Situ Brecheniauc lists: Eleri, Hunydd, Gwladys, Ceingar, Tudglid, Nyfain, Gwawr, Marchell, Lluan, Gwrygon Goddeu, Arianwen, Bethan, Ceinwen (Keyne), Cerddych, Clydai, Cynheiddon (identified with Saint Endelienta), Dwynwen, Eiliwedd, Goleudydd, Gwen, Lludd, Tudful, Tudwystl and Tybie. Other Welsh sources claim the following additional daughters: Beiol, Tydieu, Eufail, Hawystl, Edwen, Gwenrhiw, Tudwen, Callwen, Gwenfyl, Gwennan and Mwynwen.
Listed in the Life of Saint Nectan are, by his wife, Gwladys:
Adwen, Canauc (Cynog), Cleder (Clether), Dilic (Illick), Endelient (Endelienta), Helie, Johannes (Sion), Iona, Juliana (Ilud), Kenhender (Cynidr), Keri (Curig), Mabon (Mabyn), Menfre (Menefrewy), Merewenne (Marwenna), Morewenna (Morwenna), Nectanus (Nectan), Tamalanc, Tedda (Tetha), Wencu (Gwencuff, Gwengustle, name of Saint Nennocha), Wenheden (Enoder), Wenna (Gwen), Wensent, Wynup (Gwenabwy) and Yse (Issey).
According to Robert Hunt, of the holy children that settled in Cornwall, we learn that the following gave their names to Cornish churches
- Johannes at St Ive
- Endelient at St Endellion
- Menfre at St Minver
- Tethe at St Teath
- Mabon at St Mabyn
- Merewenne at Marhamchurch
- Wenna at St Wenn
- Keyne at St Keyne
- Yse at St Issey
- Morwenna at Morwenstow
- Cleder at St Clether
- Keri at Egloskerry
- Helie at Egloshayle
- Adwen at Advent
- Lanent at Lelant
The Book of Leinster lists the following sons by Brychan's wife, Dína daughter of the King of the Saxons: Mo-Goróc, Mo-Chonóc (Cynog), Diraid, Dubán (Dyfnan), Cairinne (Caian), Cairpre, Iast, Ellóc (Dilic), Paan, Cáemán and Mo-Beóc,
Breton tradition says that Brychan married Menedoc daughter of Constantine, King of the Scots. Together they were the parents of Saint Nennocha.