Carroll joined the Society of Jesus (the "Jesuits") as a postulant at the age of 18 in 1753.
 In 1755, he began his studies of philosophy and theology at Liège.
 After fourteen years, he was ordained to the diaconate and later the priesthood in 1761.
Carroll was formally professed as a Jesuit in 1771.
Carroll remained in Europe until he was almost 40, teaching at St-Omer and Liège.
He also served as chaplain to a British aristocrat traveling on the continent.
When Pope Clement XIV suppressed the Society of Jesus in 1773 in Europe, Carroll made arrangements to return to Maryland.
The brief suppression of the Jesuits was a painful experience for Carroll, who suspected the Congregation for the Propagation of the Faith of being responsible for this ill-informed decision.
 As a result of laws discriminating against Catholics, there was then no public Catholic Church in Maryland.
Carroll worked as a missionary in Maryland and Virginia.

In 1774 Carroll founded St. John the Evangelist Parish at Forest Glen (Silver Spring) on land that belonged to his mother.
In 1776, the Continental Congress asked Carroll, along with his cousin, delegate Charles Carroll, fellow Marylander Samuel Chase (1741–1811), and Benjamin Franklin (1705/06–1790), to travel north to Montreal in the Saint Lawrence River Valley to try to persuade the French Canadian-majority Province of Quebec to join the Revolution with the lower Thirteen Colonies.
The French Canadians had been forced to cede control of their territory in 1763 to the occupying British Army, which won the Seven Years' War, known as the French and Indian War in North America.
The Quebec Act of 1774 allowed French Canadians to keep their language, their religion, and much of their law.
The group was unsuccessful, but Carroll became known to other early founders of the United States Republic.
Snubbed by the local clergy on the orders of Jean-Olivier Briand, Bishop of Quebec, Carroll took an early opportunity to accompany the ailing Franklin back to the colonial capital at Philadelphia.

The Jesuit fathers, led by Carroll and five other priests, began a series of meetings at White Marsh, at what is now the site of Sacred Heart Church in Bowie, Maryland (Prince George's County, Maryland), beginning on June 27, 1783.
Through these General Chapters, they gradually organized the Catholic Church in the United States at White Marsh.
Carroll joined the Society of Jesus (the "Jesuits") as a postulant at the age of 18 in 1753.  In 1755, he began his studies of philosophy and theology at Liège.  After fourteen years, he was ordained to the diaconate and later the priesthood in 1761. Carroll was formally professed as a Jesuit in 1771. Carroll remained in Europe until he was almost 40, teaching at St-Omer and Liège. He also served as chaplain to a British aristocrat traveling on the continent. When Pope Clement XIV suppressed the Society of Jesus in 1773 in Europe, Carroll made arrangements to return to Maryland. The brief suppression of the Jesuits was a painful experience for Carroll, who suspected the Congregation for the Propagation of the Faith of being responsible for this ill-informed decision.  As a result of laws discriminating against Catholics, there was then no public Catholic Church in Maryland. Carroll worked as a missionary in Maryland and Virginia.
In 1774 Carroll founded St. John the Evangelist Parish at Forest Glen (Silver Spring) on land that belonged to his mother. In 1776, the Continental Congress asked Carroll, along with his cousin, delegate Charles Carroll, fellow Marylander Samuel Chase (1741–1811), and Benjamin Franklin (1705/06–1790), to travel north to Montreal in the Saint Lawrence River Valley to try to persuade the French Canadian-majority Province of Quebec to join the Revolution with the lower Thirteen Colonies. The French Canadians had been forced to cede control of their territory in 1763 to the occupying British Army, which won the Seven Years' War, known as the French and Indian War in North America. The Quebec Act of 1774 allowed French Canadians to keep their language, their religion, and much of their law. The group was unsuccessful, but Carroll became known to other early founders of the United States Republic. Snubbed by the local clergy on the orders of Jean-Olivier Briand, Bishop of Quebec, Carroll took an early opportunity to accompany the ailing Franklin back to the colonial capital at Philadelphia.
The Jesuit fathers, led by Carroll and five other priests, began a series of meetings at White Marsh, at what is now the site of Sacred Heart Church in Bowie, Maryland (Prince George's County, Maryland), beginning on June 27, 1783. Through these General Chapters, they gradually organized the Catholic Church in the United States at White Marsh.
John Carroll (archbishop of Baltimore)