She became a Board member of the Irish Section of Amnesty International in 1975, was elected Chair from 1983 -1987 and in 1988 became its Director.

She founded Front Line Defenders in 2001 to focus specifically on the protection of human rights defenders at risk. As Executive Director from 2001-2016, Mary had a key role in the development of Front Line Defenders into the prominent international organisation it is today.

On 1 May 2020, she took up the mandate of the Special Rapporteur on the situation of human rights defenders, where she has adopted a people-centred approach to the mandate.

She is also currently an Adjunct Professor of Business and Human Rights in the Centre for Social Innovation (CSI), School of Business, Trinity College Dublin. She is a member of the Advisory Board of both the School of Business of Trinity and the Centre for Ethics in Public Life, School of Philosophy, University College Dublin.

Among her awards are the French insignia of Chevalier de l’Ordre National de la Légion d’Honneur and the Franco-German Award for Human Rights and the Rule of Law.

She was awarded Honorary Doctorates in Law from University College Dublin and Trinity College Dublin.