She wrote for Irish immigrants in both the United States and Canada, encouraging them to attend mass and retain the Catholic faith.

[3] Sadlier showed early literary promise, publishing her first poems in the London periodical "La Belle Assemblée" at the age of 18.

[3] Following her father's death, 24-year-old Mary Anne Sadlier emigrated to North America in 1844, initially settling in Montreal, Canada.

[3] Publication records show that she contributed poems, short articles, and sketches to The Garland on a regular basis, eventually producing longer serialized works by early 1847.

Mary Anne contributed to and edited her husband's magazine and several books, while James took on the role of publisher, promoter, and advisor for much of her work.

[5][3] While living in Canada, Sadlier published 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 contributed to the Pilot and American Celt free of charge.

During her literary career, Sadlier published twenty-three novels, translated seventeen books from the French, wrote short stories and several plays.

During her stay in Montreal Sadlier also wrote two novels set in Ireland: Alice Riordan; the Blind Man's Daughter (1851) and New Lights; or, Life in Galway (1853).

Sadlier's close friend was D'Arcy McGee, a poet, Irish nationalist exile, and Canadian statesman known as one of the founding "Fathers of Confederation" who helped bring about Canada's independence.