The younger daughter of Donald Attwater, the British Catholic author and translator. John herself worked on the liturgy and history of the Catholic Church. With her father, she authored The Penguin Dictionary of Saints.
She was educated at Abergavenny Girls High School and Cambridge University, where she read English and Early History of the British Isles. She wrote several plays, including Becket and Seven Branches for Life, performed in Canterbury Cathedral in 1970 and 1975, Keepers of the Light: celebrations of the Celtic saints, performed in 1975 and 1986, and True Glory on Saint Cuthbert Mayne and his times. Her prose works include Adam Schall, A Jesuit at the Court of China (1963) and the Saints of Cornwall (1981). John also contributed several articles to the The Chesterton Review, the quarterly journal of the G.K. Chesterton Institute for Faith & Culture, including an article about the connections between Chesterton and Arthur Quiller-Couch.
She lived in Lostwithiel and intiated the annual Lostwithiel Christmas Pageant in the 1980s
She was made a bard of the Gorsedh Kernow in 1982, taking the name Myrgh An Syns, meaning 'Daughter of Saints'.