In July 1616, Tredway entered the novitiate of the Canonesses Regular of the Lateran at their Priory of Notre-Dame-de-Beaulieu in the village of Sin-le-Noble, near Douai, in the County of Flanders, which had been established in the 13th century as a hostel for travellers and the sick.
Bishop Richard Smith, who was charged with the spiritual care of the Catholics of Great Britain, then in exile in Paris, helped them generously and may be counted a co-founder.
Carre and Tredway were also practically the founders of the Seminary of St. Gregory (now Downside Abbey) for training priests for the English mission.
A pension for English ladies and a school were attached to the new monastery, of which Tredway held the office of abbess till 1675, when illness compelled her to resign.
The priory which Tredway established continued in operation, along with the school attached to it, until the French Revolution, when the English canonesses were finally forced to flee.