He was [[ordination|ordained]] a [[deacon]] on [[April 12]], 1981, and served at various [[parish]]es of the [[Diocese of Western Pennsylvania (OCA)|Diocese of Pittsburgh and Western Pennsylvania]] - assisting the bishop, assisting the priest, and serving Typika at the bishop's direction. Dcn David also served as the Parish Education Coordinator at those parishes - including speaking on teen and adult education at retreats and lecture series' in Eastern and Western Pennsylvania Dioceses, and developing educational software in game format for use at parish and regional levels. He had anticipated serving as a deacon until the end of his secular working life, before serving as priest at a small parish. In the midst of feeling somehow unfulfilled, Karen asked if he had considered seminary and the priesthood. Dcn David felt God's calling to go to seminary, and they immediately began plans to attend St Tikhon's Seminary in 1992 - he would move to the seminary, while Karen and their four children lived with her parents for the first year.
After his first year, he was ordained a [[priest]] on [[July 31]], 1993, and was assigned as rector of St Michael's Church in Old Forge, Pennsylvania, which allowed him to continue his studies and reunite his family. Fr David graduated ''cum laude'' from St Tikhon's in 1997, before starting further studies at the University of Scranton in 1999 with Bachelors of Arts majoring in Theology and in Philosophy and graduating ''magna cum laude'' in 2003 (with awards for Academic Excellence and Excellence in Philosophy, and giving Presidential Honors in 2000, 2001 and 2003 from the University President), and graduating from a Master of Arts in Theology degree (also from the University of Scranton). He continued to serve at St Michael's until January 2006, when he was transferred to the pastorate of Holy Trinity Church in Pottstown, Pennsylvania, and was then transferred from there to be Acting Rector of St Nicholas Orthodox Church, Bethlehem, PA. He also served as [[dean]] of the Philadelphia Deanery, as Treasurer of St Tikhon's Alumni Association, Secretary of the NEPA Chapter of Alpha Sigma Nu Alumni, Secretary of the Clergy Brotherhood of NEPA, Staff of ''Your Diocese Alive in Christ'' magazine, auditor of the [[OCA]] and the Diocese of Eastern Pennsylvania, and on the Bishop's Task Force on Missions and Evangelism for the Diocese of Eastern Pennsylvania.
In 1997, he received a Master of Divinity degree from [[St. Tikhon's Orthodox Theological Seminary|St. Tikhon’s Seminary]] in South Canaan, Pennsylvania. In 2003, he graduated from the University of Scranton with Bachelors degrees in Theology and Philosophy. In 2005, he also received his Master of Arts degree in Theology from the University of Scranton.
Between 2003-04, Fr. David served as a graduate assistant in the Theology Department of the University of Scranton, before serving from 2005 as Adjunct Lecturer in Philosophy, Doctrine, Church History, Comparative Theology and Modern Belief at [ [St. Tikhon's Orthodox Theological Seminary|St. Tikhon’s Seminary] ], and from March 2009 as adjunct lecturer in theology and philosophy at the Melrose Park Campus of Alvernia University in Philadelphia, Pennsylvania.
August 8]], 2007, Fr. David's wife, Matushka Karen, fell asleep in the Lord. <ref>[http://doepa.org/ news_070809_1.html] In Memoriam: Matushka Karen Mahaffey</ref> In 2009, he was assigned rector of St. Nicholas Church in Bethlehem, Pennsylvania.
On [[September 15]] 2012 he was nominated by the delegates to the Assembly of the [[Diocese of Alaska (OCA)|Diocese of Alaska]] as the candidate for election as Bishop of Sitka and Alaska by the [[Holy Synod]]. On October 9-11, 2012 the Holy Synod of OCA accepted Archbishop [[Benjamin (Peterson) of San Francisco|Benjamin’s]] presentation of Hieromonk David (Mahaffey) as the nominee for the Episcopal See of Alaska<ref>[http://oca.org/news/headline-news/holy-synod-of-bishops-concludes-fall-session Holy Synod of Bishops concludes fall session]</ref>. On Friday, February 21, 2014, Hieromonk David was consecrated Bishop of Alaska during a Hierarchical Liturgy by [[Metropolitan]] [[Tikhon (Mollard) of Washington|Tikhon of Moscow]], Archbishop [[Benjamin (Peterson) of San Francisco|Benjamin of San Francisco]], Bishop [[Michael (Dahulich) of New York|Michael of New York]], Bishop [[Irineu (Duvlea) of Dearborn Heights|Irineu of Dearborn Heights]], and Bishop [[Irénée (Rochon) of Quebec City|Irénée of Quebec City]] at St Innocent Cathedral in Anchorage, Alaska. Co-serving the bishops were [[Chancery office of the Orthodox Church in America|Chancellor for the Orthodox Church in America]] [[Archpriest]] [[John A. Jillions]], Archpriest Victor Nick ([[Chancellor]] of the Diocese of Alaska), and 40 other priests from around the diocese. A banquet was held in his honor that evening at the University of Alaska’s Lucy Cuddy Center in Anchorage<ref>[http://oca.org/news/headline-news/banquet-honoring-bishop-david-held-at-university Banquet honoring Bishop David held at University]</ref>.