He received his Ph.D. in Religion (Christian Social Ethics) from New York University in 1974.
He served as a professor of Theology and later Religious Studies at Manhattan College from 1966-2016.
He has taught courses on Contemporary Moral Issues, Religious Dimensions of Peace, and the Labor Studies Colloquium.
Fahey is the co-editor of A Peace Reader (Paulist, 1990), the author of War and the Christian Conscience (Orbis Books, 2005), and edited with an Introduction to Walter Rauschenbusch: Essential Spiritual Writings (Orbis Books, 2019).
He was a co-founder of Pax Christi USA and served on the National Council of The Fellowship of Reconciliation, The Consortium on Peace Research, Education and Development (COPRED), the Peace Studies Association, and Interfaith Worker Justice.