Sadlier publishes roughly sixty novels and numerous stories in her lifetime. She writes for Irish immigrants in both the United States and Canada, encouraging them to attend mass and retain the Catholic faith. In so doing, Sadlier also addresses the related themes of anti-Catholicism, the Great Famine, emigration, and domestic work. Her writings are often found under the name Mrs. J. Sadlier.
Upon the death of her father, Francis, a merchant, Mary Madden emigrates to Sainte-Marthe, Quebec in 1844, where she marries publisher James Sadlier, also from Ireland, on November 24, 1846. Sadlier publishes much of her work in the family’s Catholic magazine, The Tablet. Sadlier experiences her most productive literary period after her marriage and is most creative after the time all of her children are born. While living in Canada, Sadlier publishes eighteen books — five novels, one collection of short stories, a religious catechism, and nine translations from the French — in addition to assorted magazine articles she contributes to the Pilot and American Celt free of charge. In New Lights (1853), Sadlier deals with the Great Famine for the first time. The book proves one of her most popular, going through at least eight editions in fifty years. In this novel, Sadlier focuses a polemical attack on the Protestant practice of converting Irish peasants by promising them soup, but condemns peasant retaliation and violence.
In the early 1860s, the couple moves to New York City. The Sadlier’s New York home becomes the hub of literary activity in the Catholic community, and Sadlier also enjoys the company of the brightest Irish writers in the United States and Canada, including New York Archbishop John Hughes, editor Orestes Brownson, and Thomas D’Arcy McGee. She holds weekly salons in her Manhattan home, as well as her summer home on Far Rockaway on Long Island. Her closest friend is D’Arcy McGee, a poet, Irish nationalist exile and Canadian statesman known as one of the founding “Fathers of Confederation” who helps bring about Canada’s independence. McGee and Sadlier share an interest in a “national poetry” that does not only capture the spirit of a people, but inspires them to political and national independence. While McGee, as a man, can take part in political rallies and organize Irish American support for Home Rule, Sadlier, as a woman, directs her support for Irish independence into literature. McGee’s biographer notes that Sadlier’s success inspires him to write emigrant novels, and is planning a novel on this subject at the time of his assassination by an Irish American radical in 1868. His death is a crushing blow to Sadlier and her husband. Sadlier edits a collection of McGee’s poetry in 1869 in tribute to his memory.
Sadlier remains in New York for nine years before returning to Canada, where she dies in Montreal on April 5, 1903. In later years she loses the copyright to all her earlier works, many of which remain in print. One of Mary Anne’s daughters, Anna Theresa Sadlier, also becomes a writer.