[5] He held academic posts at Oxford, Trinity & All Saints College, Liverpool Hope University, and Leuven (Belgium).
He was also Professor of Theology and Director of the Center for Catholic Thought and Culture at the University of San Diego.
[6] In January 2014, Mannion moved to Georgetown University, became Joseph and Winifred Amaturo Chair in Catholic Studies at the Department of Theology and Religious Studies, and he was a senior research fellow at the Berkley Center, focusing on the church's role in the world, social ethics, and ecumenical and inter-religious dialogue.
[6] He was made an honorary fellow of the Australian Catholic University, senior research fellow in public theology at the "Centro per le Scienze Religiose of the Fondazione Bruno Kessler," a 2004 Coolidge Fellow at Union Theological Seminary / Columbia University, and was chosen to take part in the "Teaching the Ethical, Legal and Social Implications of the Human Genome Project" project at Dartmouth College’s Ethics Institute.
[9] He completed six years as founding co-chair of the Ecclesiology Program Unit of the American Academy of Religion (the world's largest scholarly organisation in the field) and sat on the steering groups of both the AAR's Ecclesiology and Vatican II Studies Program Units.