Catholic life in Maine predates the 19th century. In 1604, Pierre du Guast Sieur de Monts, a Huguenot, led about 80 French Catholics and Protestants to settle on Ste. Croix Island in the Ste. Croix River near present-day Calais. A brutal winter claimed nearly half the colonists, including their priest, and the site was abandoned the following year. Still, French Catholics established later settlements on Mount Desert Island (1613), Castine (1633), and Augusta (1646). 

Jesuit missionaries played a central role in spreading the faith. Blessed François Montmorency Laval, vicar apostolic of New France, recorded more than 200 baptisms at the Assumption Mission in Augusta between 1660 and 1663. Missionary work continued despite conflicts between France and England, culminating in the destruction of the Native village at Norridgewock in 1724, where Father Sebastian Rale, S.J., was killed after decades of ministry.

During the American Revolution, Native communities in Maine were visited by French naval chaplains allied with the colonists. In 1785, displaced Acadians resettled in the Madawaska region, laying the foundation for Catholic parishes along the St. John River Valley.

By 1798, Bishop John Carroll of Baltimore sent Father Jean Lefebvre de Cheverus to minister at Indian Island and to found St. Patrick Parish in Newcastle. New parishes quickly followed in North Whitefield, Eastport, Machias, and Portland, serving a growing wave of Irish immigrants. In 1834, Bishop Benedict Fenwick of Boston established a Catholic farming community at Benedicta with the hope of creating a college there. Anti-Catholic prejudice, however, brought violence in the 1850s, including the burning of churches in Bath and Lewiston and the mob assault of Father John Bapst, S.J., in Ellsworth.

The Diocese of Portland’s first appointed bishop, Rev. Henry B. Coskery, declined the position in 1853. Two years later, Bishop David W. Bacon (1815–1874) of Brooklyn, New York, arrived quietly in Portland, disguised as a layman to avoid unrest. Under his leadership, the first Catholic schools opened in Portland, Bangor, and North Whitefield. Despite setbacks from the Civil War and the Great Fire of Portland (1866), the Cathedral of the Immaculate Conception was completed in 1869. By 1870, jurisdiction for the Madawaska region, secured by the Webster-Ashburton Treaty, was transferred from New Brunswick to Portland.

Bishop James Augustine Healy (1830–1900), the first African-American Catholic bishop in the United States, succeeded Bishop Bacon in 1875. During his 25 years of service, he oversaw a period of remarkable growth, founding 33 parishes, 22 schools, 18 convents, and several hospitals and orphanages. Known as a champion of children and the poor, Bishop Healy guided the diocese during a time of rapid immigration from Ireland and French Canada.

He was followed in 1901 by Bishop William H. O’Connell (1859–1944), remembered for his diplomatic mission to Japan before being appointed Archbishop of Boston in 1906. His successor, Bishop Louis Sebastian Walsh (1858–1924), faced waves of new immigrants from Poland, Italy, Slovakia, and Lithuania. Tensions with Franco-Americans over parish property ownership marked his tenure, yet he left a legacy as a historian and strong supporter of the National Catholic Welfare Conference.

Bishops John Gregory Murray (1877–1956) and Joseph Edward McCarthy (1876–1955) guided the diocese through the Great Depression, World War II, and significant school construction. Bishop Daniel J. Feeney (1894-1969) ushered in the Second Vatican Council reforms, celebrating the centennial of Bishop Bacon’s arrival and expanding Catholic ministries across Maine.

The second half of the 20th century brought new challenges. Bishop Peter Leo Gerety (1912-2016) emphasized parish growth, social services, and ecumenical outreach before being appointed Archbishop of Newark. Bishop Edward C. O’Leary (1920-2002) continued this momentum, appointing women to diocesan leadership roles and fostering relationships with other Christian communities.

Benedictine Bishop Joseph J. Gerry (1928–2023) deepened pastoral teaching through annual letters and shepherded the diocese through the painful revelations of clergy sexual abuse. The Diocese established its first Review Board in 1993 and implemented victim support processes that remain in place today.

At the turn of the century, the diocese celebrated the Jubilee Year 2000 with a Eucharistic Congress in Augusta and completed a major renovation of the Cathedral of the Immaculate Conception. 

In February 2004, Bishop Richard Malone was installed as the 11th Bishop of Portland. He was succeeded in 2014 by Bishop Robert P. Deeley, J.C.D., the 12th bishop. In 2024, Bishop James Ruggieri was named the 13th Bishop of Portland, continuing the diocese’s legacy of faith and service.