The Cathedral is the Mother Church of the Diocese of Portland. Its Pastor is the Bishop of Portland. Its Rector is the Very Reverend Seamus Griesbach (2021 - present). The term "cathedral" distinguishes it from other churches as being the Seat ('cathedra') of the Bishop of the diocese.

Construction of the original Cathedral began in 1866 under the direction of noted New York architect, Patrick C. Keeley. It is acknowledged as his finest work in Maine. The walls were nearly completed when the July 4, 1866 fire destroyed it to the ground. Rebuilding began in 1868 and the Cathedral was dedicated on September 8, 1869.

The Cathedral is on the National Register of Historic Places (1985), the Greater Portland Landmarks (1984), and has been designated a National Catholic Historical Site by the International Order of Alhambra (2003).

It is a magnificent example of neo-Gothic architecture, restored in 2000 (the Millennium Restoration) as nearly as possible to its original splendor. A prior restoration took place in 1921 and a renovation in 1969 (following Vatican II).

The interior is 186' by 70' and the vaulted ceiling rises 70' above the floor. The length of the main aisle is 150'. The Cathedral seats 900 people. There are 7 imposing pillars on each side of the nave which form 7 Gothic arches.

The Cathedral has three steeples, the highest of which soars 204' high, the tallest structure in Portland, overlooking Portland Harbor and Casco Bay to the east and the New Hampshire mountains to the west