After Catherine McAuley died, Mother de Pazzi took over as Superior at Baggot Street, and Sister M. Cecilia Marmion was elected as her assistant. Cecilia assisted de Pazzi in establishing new foundations in Liverpool, Newfoundland, and Queen's Square, London. Cecilia succeeded de Pazzi as Superior of Dublin in 1844. Cecilia Marmion answered Bishop John Brady’s request in Perth, Western Australia, to send Sisters to his diocese. The mission led by Mother M. Ursula Frayne in 1846 included a group of six Sisters. In the same year, Cecilia informed another Mercy group that Bishop Hughes of New York was looking for sisters to serve in his diocese. This resulted in Mother M. Agnes O'Connor bringing a foundation to New York in 1846. Although Cecilia did not lead a new foundation, she was crucial in sending Sisters to England, Australia, and North America.

Cecilia was one of three daughters of Francis and Mary Marmion who joined Catherine McAuley’s newly formed congregation. The family was well educated and provided significant assistance to Catherine. Cecilia, who was artistic, translated various works from French to English. She joined the Sisters of Mercy on Baggot Street, where Catherine McAuley served as her superior and sometimes novice mistress.

Catherine McAuley clearly held Cecilia Marmion in high esteem. She appointed her as Novice Mistress at Baggot Street. Cecilia accompanied the foundress on some of her journeys. Cecilia's mission was mostly in education, leadership, and spiritual guidance. She recognised and shared Catherine's aspirations and missionary vision and was instrumental in spreading the Mercy Order to two other continents.

Being a timid person, Cecilia found traveling to be quite difficult. Magdalena Frisby recounts that she was also highly health-conscious, which is not surprising given that her two sisters who joined her perished at a young age.

The rapid spread of the Mercy Order was accelerated by Cecilia Marmion’s dedication to establishing new foundations. These Mercy foundations had a significant impact on education, healthcare, and social services in Australia and the United States of America.

Sullivan, Mary C, ed. The Correspondence of Catherine McAuley 1818 - 1841. Dublin, Co Dublin, Ireland: Four Courts Press Ltd, 2004.