On 12 December, 1831, Catherine McAuley, with two companions, made religious profession in the Presentation Convent of Georges Hill, Dublin where they had spent the previous year of the novitiate. On that day Archbishop Daniel Murray formally blessed the first Convent of Mercy at the House of Mercy in Baggot Street which Catherine had built with her foster parents' legacy and opened in 1827. He appointed Catherine as superior and a month later received as the first Mercy novices seven of Catherine's helpers who had worked with her from the beginning.
Catherine's concept of a Religious Institute included an availability for ministry not typical in that era. Feeling themselves called to serve Christ as the needs of His poor demanded, and encouraged by the Archbishop, Catherine and her companions took as their special works the instruction of poor girls, visitation of the sick and the protection of distressed women of good character.
The 'walking nuns', who were without the usual strict enclosure, and whose convents quickly became part of many dioceses, inspired local girls to see and meet local needs, causing the new Institute to spread rapidly. By Catherine's death in 1841, there were ten houses of Mercy in Ireland. In the ten years of her religious life, twelve autonomous foundations of Sisters of Mercy, all following the same Rule, had been established in Ireland and England.
In 1841, the year of Catherine's death, the original Rule and Constitutions was formally approved by Pope Gregory XVI. The Institute spread to other continents, each foundation being given local autonomy to enable the sisters to answer appropriately the local needs. The instruction of poor girls broadened to include the education of boys and of adults. Parochial schools, select schools, boarding schools, industrial schools, schools of nursing education became part of the Mercy scene. Visitation of the sick widened into care of the sick whether in hospitals, their own homes or homes of the aged, as well as into care of those whose sickness or poverty was spiritual more than physical. Homes for those deprived of normal family life opened their doors to children, the aged, the handicapped and the disturbed.