Gwenllian was the daughter of Maredudd ap Owain and Elen de Valle. The date and place of her birth have not been found.
Her husband was Maredudd ap Cadwgon. They were married, but the date and place have not been found. Their two known children were Ellyw Angharad verch Maredudd (?-?) and Joan verch Maredudd (?-?).