Fenwick was appointed the second Bishop of Boston by Pope Leo XII on May 10, 1825, succeeding Jean-Louis Lefebvre de Cheverus.
The papal bull notifying him of his appointment arrived in July 1825, and he embarked on an eight-day spiritual retreat.
Upon its completion, Fenwick was consecrated a bishop in the Cathedral of the Assumption of the Blessed Virgin Mary in Baltimore on November 1.
Archbishop Ambrose Maréchal served as principal consecrator, while Bishops John England and Henry Conwell were co-consecrators.
Fenwick arrived in Boston on December 3, and formally took canonical possession of the Diocese of Boston at the original Cathedral of the Holy Cross, on December 21, 1825.

Though the ecclesiastical jurisdiction of the diocese encompassed all of New England, Bishop Fenwick had only two priests under his charge, who served three Catholic churches, besides the cathedral, in all of New England: Saint Augustine's Chapel in Boston, St. Patrick's Church in Newcastle, Maine, and a small church in Claremont, New Hampshire.
Throughout New England, there were approximately 10,000 Catholics.
Due to significant Irish immigration, the Catholic population in the diocese grew to at least 30,000 by 1833.
Fenwick traveled throughout the large territory to manage the diocese and administer the sacrament of confirmation.
This included visiting Penobscot and Passamaquoddy tribes in Maine, who were largely Catholic, and were the subject of intensive proselytism by Protestant evangelists.
Fenwick ordered the construction of St. Anne's Church in Old Town, Maine for them in 1828, and sought to improve their schools.

Fenwick addressed a shortage of priests in his diocese by sending prospective seminarians to Maryland and Canada to be educated, and by incarnating several priests from other dioceses.
He also trained several students in a makeshift seminary at his episcopal residence.
As a result, the number of priests in the diocese had increased to 24 by this time.
Many new parishes were founded throughout New England.
Fenwick attended the First Provincial Council of Baltimore convened in 1829.
As in South Carolina, Fenwick was an ardent opponent of lay trusteeism in the Diocese of Boston.
With a rapidly expanding Catholic population in the diocese, a portion of the territory was removed to form the Diocese of Hartford in 1843.
That year, John Bernard Fitzpatrick was appointed as Fenwick's coadjutor bishop, and would later succeed him as Bishop of Boston.

By the end of Fenwick's episcopate, the number of Catholics in the Diocese of Boston (after the removal of Hartford) had increased to 70,000, in addition to 37 priests, and 44 churches.
In December 1845, Fenwick's health began to decline, due to an ailment of the heart.
Eight months later, he died on August 11, 1846.
After the funeral, his body was carried from the Cathedral of the Holy Cross to the train station, from where it was taken to the College of the Holy Cross and buried in the cemetery.
Bishop Fenwick High School in Peabody, Massachusetts, which opened in 1959, was named in his honor.
Fenwick was appointed the second Bishop of Boston by Pope Leo XII on May 10, 1825, succeeding Jean-Louis Lefebvre de Cheverus. The papal bull notifying him of his appointment arrived in July 1825, and he embarked on an eight-day spiritual retreat. Upon its completion, Fenwick was consecrated a bishop in the Cathedral of the Assumption of the Blessed Virgin Mary in Baltimore on November 1. Archbishop Ambrose Maréchal served as principal consecrator, while Bishops John England and Henry Conwell were co-consecrators. Fenwick arrived in Boston on December 3, and formally took canonical possession of the Diocese of Boston at the original Cathedral of the Holy Cross, on December 21, 1825.
Though the ecclesiastical jurisdiction of the diocese encompassed all of New England, Bishop Fenwick had only two priests under his charge, who served three Catholic churches, besides the cathedral, in all of New England: Saint Augustine's Chapel in Boston, St. Patrick's Church in Newcastle, Maine, and a small church in Claremont, New Hampshire. Throughout New England, there were approximately 10,000 Catholics. Due to significant Irish immigration, the Catholic population in the diocese grew to at least 30,000 by 1833. Fenwick traveled throughout the large territory to manage the diocese and administer the sacrament of confirmation. This included visiting Penobscot and Passamaquoddy tribes in Maine, who were largely Catholic, and were the subject of intensive proselytism by Protestant evangelists. Fenwick ordered the construction of St. Anne's Church in Old Town, Maine for them in 1828, and sought to improve their schools.
Fenwick addressed a shortage of priests in his diocese by sending prospective seminarians to Maryland and Canada to be educated, and by incarnating several priests from other dioceses. He also trained several students in a makeshift seminary at his episcopal residence. As a result, the number of priests in the diocese had increased to 24 by this time. Many new parishes were founded throughout New England. Fenwick attended the First Provincial Council of Baltimore convened in 1829. As in South Carolina, Fenwick was an ardent opponent of lay trusteeism in the Diocese of Boston. With a rapidly expanding Catholic population in the diocese, a portion of the territory was removed to form the Diocese of Hartford in 1843. That year, John Bernard Fitzpatrick was appointed as Fenwick's coadjutor bishop, and would later succeed him as Bishop of Boston.
By the end of Fenwick's episcopate, the number of Catholics in the Diocese of Boston (after the removal of Hartford) had increased to 70,000, in addition to 37 priests, and 44 churches. In December 1845, Fenwick's health began to decline, due to an ailment of the heart. Eight months later, he died on August 11, 1846. After the funeral, his body was carried from the Cathedral of the Holy Cross to the train station, from where it was taken to the College of the Holy Cross and buried in the cemetery. Bishop Fenwick High School in Peabody, Massachusetts, which opened in 1959, was named in his honor.
Benedict Joseph Fenwick