For his regency, Prendergast taught Latin and Greek at Loyola High School in Montreal from 1967-1969 and, after ordination and graduate studies in theology, taught in Halifax at the Atlantic School of Theology from 1975–1981, then was Rector of Toronto's Regis College from 1981–87, and its Dean of Theology from 1991-1994. From 1992-94 he assisted Frederick Henry, then the Auxiliary Bishop of London (Ontario), later Bishop of Thunder Bay and Bishop of Calgary, in conducting an apostolic visitation of the English-language seminaries in Canada for the Vatican. In 1995, he was a Visiting Professor at the Ecole Biblique in Jerusalem.
In 1998 Prendergast was appointed the Archbishop of Halifax, and installed on 14 September 1998. He served ex officio as the Chancellor of Saint Mary's University, Halifax, a position granted to the Archbishop of Halifax until 2007, when the archdiocese and the university agreed to bestow the title of Visitor instead.
Prendergast was named the ninth Archbishop of Ottawa by Pope Benedict XVI on 14 May 2007[2] and was installed on 26 June 2007. In this office, he also serves ex officio as Chancellor of Saint Paul University. In 2012, Prendergast welcomed a number of members of the Anglican Catholic Church of Canada into full communion with the Roman Catholic Church.[3]
He was appointed Apostolic Administrator of Alexandria-Cornwall on January 13, 2016 and became bishop there on April 27, 2018 while remaining Archbishop of Ottawa. The two dioceses were united “in persona episcopi” (“in the person of the Bishop”). They were then united on May 6, 2020 to form the Archdiocese of Ottawa-Cornwall, under him as archbishop.
On 30 November 2020, he was appointed apostolic administrator of Diocese of Hearst–Moosonee, after the resignation of its bishop, Robert Ovide Bourgon. In the following 4 December, he formally retired as archbishop, and was succeeded by Marcel Damphousse, the coadjutor archbishop who was appointed when the new Archdiocese of Ottawa-Cornwall was formed.
Other episcopal roles
After serving on the Canadian Bishops' Commission for Relations with Associations of Clergy, Consecrated Life and Laity, as a member of the CCCB Theology Commission [now known as the Doctrine Commission], its Commission on Social Communications, several terms on the Permanent Council (including since 2015 as representative of the Bishops of Ontario) and earlier as co-chair of the national Anglican-Roman Catholic dialogue, Prendergast is, since October 2014, a member of the CCCB's Episcopal Liaison Committee to the Canadian Catholic Organization for Development and Peace.
His other responsibilities include president of National Evangelization Teams Canada (NET Canada) and chair of CNEWA (Catholic Near East Welfare Association) Canada, both of which are based in Ottawa.
Prendergast is a teacher, writer and retreat master. From 1994 to 2005, he wrote a weekly column in the Catholic Register. A collection of his essays was published as Living God's Word: Reflections on the Sunday Readings for Year A, B, C.[9] He has been a writer or editor for several journals, and lectured and given retreats across Canada, in the United States and Rome.
Prendergast was Apostolic Administrator of the Diocese of Yarmouth, Nova Scotia (now part of the Halifax-Yarmouth Archdiocese) from January 2002 to May 2007, and took part in the Apostolic visitation to Ireland which began in 2010 and was completed in 2011.