In 1917 Tillyard came out as a mystic. Having already begun to record her mystico-spiritual experiences and their psychophysical manifestations in detailed diaries intended for posthumous publication, she also began to transcribe them in more or less fictionalised form in novels, homiletic books and moralistic short stories written between 1917 and 1958, some published, some not. In 1926 she wrote a biography of her aunt, Agnes Elizabeth Slack who campaigned for temperance. She documenting her aunt's travels to speak about temperance in Ireland, Canada, America, Scandinavia and South Africa.