Myfanwy was the daughter of Madog ap Cynwrig Fychan but her mother is unknown. The date and place of her birth have not been found.
Her husband was Ieuaf ap Adda. They were married, but the date and place have not been found. Their three known children were Adda Goch ap Ieuaf (?->1356), Hywel ap Ieuan (?-?) and Llewellyn ap Ieuaf (?-?).