Cassidy was ordained in the Roman Catholic Church as a deacon in 1985 and as a priest in 1986. After ordination to the priesthood, he became a member of the retreat team at Loyola House, Guelph, Canada, where, among other things, he directed thirty-day retreats. He transferred to Montreal to care for an ailing parent, becoming co-director of the Ignatian Centre, Montreal, and a visiting professor at Concordia University where he taught courses in the history of spirituality and social ethics. During these years, he began a doctorate in Ottawa and directed dozens of workshops and retreats across Canada and the United States.