Priesthood

Hennessey was ordained to the priesthood by Cardinal Humberto Medeiros at the Cathedral of the Holy Cross in Boston on May 20, 1978 for the Archdiocese of Boston.^[3][4]

After his ordination, the archdiocese assigned Hennessey as parochial vicar at St. Joseph Parish in Hanson, Massachusetts. Hennessey's next assignment was as parochial vicar at St. Peter Parish in Plymouth, Massachusetts. In 1983, he went assigned to St. Joseph Parish in Needham, Massachusetts, staying there for three years. Hennessey then entered a graduate studies program at Moreau Seminary at the University of Notre Dame in Notre Dame, Indiana.^[5][1]

Hennessey spent the next six years serving in the Missionary Society of St. James the Apostle in a Bolivian parish with a Catholic population of 45,000. Following his return to Boston in 1994, the archdiocese named Hennessey as administrator of Most Holy Redeemer Parish in East Boston, Massachusetts, a largely Hispanic parish, where he served for the next 12 years. In 1995, he also became the administrator of Our Lady of the Airways Chapel at Logan International Airport in Boston for the next three years.^[5][1]

Auxiliary Bishop of Boston

On October 12, 2006, Pope Benedict XVI appointed Hennessey as an auxiliary bishop of Boston and titular bishop of Tigias. He received his episcopal consecration on December 12, 2006, from Archbishop Seán O'Malley, with Bishops John Boles and Emilio Allué serving as co-consecrators^[6][4] Hennessey's episcopal motto is "Magnificat Anima Mea Dominum" Luke 1:46-55, meaning, "My soul doth magnify the Lord".^[6]

In January 2009, Hennessey addressed a group of parish members in the Dorchester neighborhood of Boston. He warned them that a decline in mass attendance in their churches and the shortage of priests in the archdiocese could result in the merging of parishes in Dorchester.^[7]

Hennessey first held the post of episcopal vicar for the Central Pastoral Region of the archdiocese, but was transferred on January 23, 2014 to the Merrimack Region.^[6][3] In December 2014, the outdoor nativity scene at Sacred Heart Church in Haverhill, Massachusetts, was vandalized; someone stole the baby Jesus figure and replaced it with the head of a pig. Hennessey later blessed the nativity scene and expressed his hope that the vandals seek counseling.^[8]

In the United States Conference of Catholic Bishops, Hennessey has served as a member of the Subcommittee on the Catholic Campaign for Human Development.^[9] As of 2023, Hennessey is the vicar general regional bishop for the Merrimack Region.^[10]