Hayden is educated initially at the Dominican College, Eccles Street and then at Alexandra College in Dublin. She attends the Royal University of Ireland where she graduates with a BA in 1885 and an MA in 1887 in Modern Languages. With Agnes O’Farrelly she campaigns for women’s rights in the university.
A campaigner for gender equality and noted as a public speaker, Hayden is a prominent member of the Irish Women’s Suffrage and Local Government Association. She is a member of the Gaelic League and friends with Patrick Pearse. However, she opposes violence and disapproves of the 1916 Easter Rising.
Hayden helps to form the Women’s Social and Progressive League as a political party committed to opposing the 1937 constitution of Ireland and any regressive consequences it would entail. She opposes articles 40, 41, and 45 concerning the status of women.
Mary Hayden dies in Dublin on July 12, 1942.