Munitiz was born in Cardiff, Wales, of Basque parentage and educated by the Christian Brothers at Liverpool, and subsequently at the Junior Seminary at Comillas, Cantabria.
He was appointed to the staff of Leuven University with special responsibility for the Series Graeca of Corpus Christianorum (now published by Brepols).
He was subsequently retained as a research fellow at Birmingham University before retiring to Campion in 2010 and then to a house in London in 2016.
Early research work with Marcel Richard on the Series Graeca led him to publishing his thesis on Theognostos (with an academic introduction in French) volume 5 in that series, and completing Richard's volume, the Questions and Answers of Anastasios of Sinai, vol.
His interest in the work of the founder of the Society of Jesus led to articles and most notably to a joint Penguin publication on Ignatius of Loyola's Personal Writings.