Mafi was appointed coadjutor bishop of Tonga in June 2007 and was consecrated bishop in that year. When he succeeded Bishop Soane Lilo Foliaki as Bishop of Tonga on 18 April 2008, he became the first diocesan priest to be bishop of the diocese of Tonga. His three predecessors were members of the Society of Mary. He participated in the Third Extraordinary General Assembly of the Synod of Bishops on the Pastoral Challenges of the Family in the Context of Evangelization held on October 5–19, 2014. Mafi has been president of the Episcopal Conference of the Pacific since 2010.