Shane MacCarthy was born in Washington, DC, on August 19, 1938. The eldest of five boys, he attended Catholic schools, graduating from the Catholic University Campus School in 1952, Gonzaga High School in 1956, and from Holy Cross College in Worchester, Massachusetts, in 1960. MacCarthy's seminary studies at Saint Vincent's Seminary in Latrobe, Pennsylvania, occurred from 1960-1965, largely overlapping the Second Vatican Council which began in 1962 and ended the same year as his ordination in 1965. He served as a priest for the Archdiocese of Washington at St. Camillus Parish in Silver Spring, Maryland, for two years before requesting a transfer that would enable him to be involved in urban ministry through his assignment to Assumption Parish in Southeast Washington, DC where he spent the next eight years.

As a young priest, he engaged in diocesan-sponsored seminars taught by some of the leading American Roman Catholic theologians who were then faculty members at the Catholic University of America. These studies, sponsored by the Archdiocese, brought MacCarthy and his fellow priest-students into personal contact with many of the theologians - including the Rev. Charles Curran - who subsequently were themselves engaged in providing a scholarly response to Pope Paul VI's encyclical, Humanae Vitae.

Following the publication of Humanae Vitae in July 1968, as part of a group of 40 men, the vast majority of whom were serving in Archdiocesan parishes, he signed a Statement of Conscience, which expressed disagreement with the Church's assessment surrounding the encyclical's approach to artificial birth control. Some signers objected to the encyclical's upholding of the Catholic Church's condemnation of the practice; others differed on how to deal with the issue pastorally, particularly as it affected confession and reception of the sacraments. As a result of the Statement of Conscience, MacCarthy and the other signers were penalized, to varying degrees, by Patrick O'Boyle, the Cardinal Archbishop of the Archdiocese of Washington. Many, like MacCarthy, were suspended from preaching, teaching, and hearing confessions. Some few priests, particularly those among the leadership cadre, were expelled from their parish rectories. Others received less severe penalties.

MacCarthy was one of 19 priests who disputed their suspension and sought to bring their case before the Church judicially. With the advocacy of an association of American priests called the National Federation of Priests' Councils, and its Committee of Concerned Canon Lawyers, the group petitioned Pope Paul VI to arbitrate the case after it was determined that the Washington Archdiocesan Tribunal could not bring the matter to an acceptable close. Pope Paul VI set the Congregation for the Clergy the task of hearing the case. The Congregation finally reached the decision that Cardinal O'Boyle had followed the requirements for the Code of Canon Law. Eventually, the priests who still wished to resume their duties were able to do so by signing a statement crafted by Cardinal Wright that seemed to mollify the encyclical's original intent.

MacCarthy left active ministry with the Roman Catholic Church in 1975 and a year later, he accepted a nomination to serve as the Peace Corps Director in Ghana, West Africa. Upon returning to the United States in 1978, he elected to join the Agency for International Development. As a USAID Foreign Services Officer, he and his family undertook five year assignments in Swaziland, Southern Africa, and Cairo, Egypt. In each of his countries of overseas work, MacCarthy assumed weekend pastoral responsibilities as an Anglican priest prior to returning to the United States in 1996. He then served as a USAID personal service contractor, working as the Bureau for Europe and Eurasia's Outreach and Training Coordinator until his retirement on July 3, 2009.

MacCarthy married Karen Neubert in 1975 and has two children, Sean, born in 1977 and Sarah, born in 1981. A fifth-generation Washingtonian, he maintains his local roots and currently lives in McLean, Virginia.