Born as Phyllis Kirkus in York, Sister Gregory made her name as an outstanding historian of English convent life whose own biographical work has provided invaluable research tools for historians of the English recusant period. Sister Gregory presided over a collection of antique books, artefacts and manuscripts at the Bar Convent in York, the oldest convent in England, and a girls' school from 1686-1985. Well into her 90s, Sister Gregory gave talks, which were attended by professors of history, schoolchildren and even Robbie Coltrane.
She gained a place at Newnham College, Cambridge in 1929, eight years after the university voted against conferring its degrees on women. The young Phyllis Kirkus flourished in its challenging and competitive academic atmosphere, while chafing at rules aimed at ensuring modest conduct among women students, such as having to wear a hat in lectures.
She was received into the Catholic Church in the chapel of the Canonesses of Saint Augustine next to Newnham, since Fisher House, the Catholic chaplaincy under Monsignor Alfred Gilbey, was also closed to women. Training in librarianship, her application for a post at Southampton University was thwarted by a pilfering office boy who destroyed her letter. She later ascribed this disaster to divine providence, as it led to a job in the University of Hull, where she explored her growing sense of a vocation to religious life. A random meeting with Sister Philip Hardman, a scholar and historian, led her to the Bar Convent and the discovery of the Institute of the Blessed Virgin Mary (IBVM), an unenclosed order of women founded by Mary Ward (1585-1645) in 1609. Phyllis' Anglican family were not accepting of either her conversion or her decision to enter religious life, yet she did so anyway. Entering the novitiate, she took the religious name Sister Gregory.
The outbreak of World War II necessitated the evacuation of St. Mary's Convent, Hampstead, where she was teaching. Nuns and children set out together on a remarkable exodus which led them to the stately East Sussex mansion of Lady Catherine Ashburnham. Seeing the evacuees as intruders, the eccentric Lady Catherine refused them any comfort, loading logs into her quarters and the servants' hall, while the nuns and children froze in the bitter winter of 1940. After a second evacuation, the school found a permanent new home at Coombe House, Shaftesbury, in 1945. As headmistress of St. Mary's School, Shaftesbury, Sister Gregory determined to run an exemplary school for women of the future.
In 1972 she was appointed as Provincial Superior of the Institute of the Blessed Virgin Mary, later known as the Congregation of Jesus. This was during the turmoil of the Second Vatican Council (1962-1965). Sister Gregory did not welcome the changes proposed by the council, given her conservatism. With the help of educator (and Catholic convert), Dr Margaret Wileman, Sister Gregory administered educational programmes for her own province and two international summer schools where sisters from around the world, responsible for running schools around the world. Her influence as an educator encouraged improvements which enabled the Bar Convent Grammar School, and St Mary's Schools in Hampstead, Ascot, Shaftesbury and Cambridge, to successfully evolve to schools administered by laypersons.
Around 1981 set up the Bar Convent Museum, archives and library. She wrote numerous sketches of school and convent life in the seventeenth and eighteenth centuries for the Catholic Record Society. A lifelong teetotaller who lived in considerable personal austerity, it was only when she appeared on a Channel Four documentary on the Bar Convent that her astonished sisters discovered her to be an avid buyer of lottery tickets, in the hope of winning the fortune that would secure the convent's future.
She died, aged 96, on the feast-day of Saint Margaret Clitherow, like Ward and Kirkus, a Yorkshirewoman, whose relics are kept in the Bar Convent chapel. Shortly before her death, Sister Gregory had completed a biography of Mother Mary Ward's IBVM, which was to be published in 2009 to mark the 400th anniversary of the order.