Mary Morrissy was educated at the Rathmines School of Journalism. She worked in Australia, and as a sub-editor of The Irish Press. She has taught creative writing for the University of Arkansas, and University of Iowa creative writing summer programmes. In 1995, she was awarded a Lannan Literary Award for Fiction.
In 2008 – 09, she was Jenny McKean Moore Writer in Washington at George Washington University, Washington DC. Morrissy was a Cullman Fellow at the New York Public Library, for her work-in-progress, The Duchess, an imagined autobiography of Bella O’Casey, the sister of Seán O’Casey. The novel was published in 2013 as The Rising of Bella Casey.
In 2015, she was appointed as Lecturer in Creative Writing at University College Cork.