The church was built in 1842 to minister to the growing Irish population of old West Baltimore, who had immigrated to the city in vast numbers to work for the B&O Railroad.

[2] Robert Cary Long, Jr., a prominent local "ecumenical architect," (both secular and religious) and son of a similarly famous accomplished father of the same name[3] designed the church in the Greek revival style with imposing granite pillars gracing the facade of the building.

The inscription above the tabernacle read, "Tu Es Petrus," a reference to the biblical passage, "You are Peter, and on this rock I will build my Church" (Matt.

The older parish had been founded in 1770 as the first "Roman Catholic Congregation of Baltimore Town" with new immigrants of Acadians, who were French Canadians expelled recently from their colony of Acadia, now the British-occupied province of Nova Scotia after the losses by France in North America from the French and Indian War (known in Europe as the Seven Years' War, 1756–1763) and settled along the southwestern shores of "The Basin" (now Inner Harbor) of the Patapsco River which became known as "Frenchtown," beyond the intersection of Forest (South Charles) and King George (now West Lombard) Streets.

With the joining of some additional Irish settlers, the Roman Catholic population of the new Baltimore Town was growing with the first Mass being said by the Rev.

Father John Carroll was named the first bishop for the United States in 1789, installed the following year as Ordinary of the new Diocese of Baltimore, "The Premier See."

Gradually, the members of the parish began moving northwestward and to the west for other residential areas being constructed, as the center became increasingly filled with stores, shops and manufactories.

The new neighborhood in inner West Baltimore later acquired the name of ""Union Square" for the prominent small landscaped park in its center.

That joined a series of other parks and squares such as Arlington and Franklin which dotted the residential landscape of the western side of the burgeoning city as grids of rowhouses were extended north and south as it grew westward.

[2] In addition to their educational duties, the Sisters also assisted the poor and sick of the community by opening the "House of Mercy for distressed women".

The limits of the parish grew so much in the early years by 1865 that a second church was established (St. Martin's at Fulton Avenue and West Fayette Street) and dedicated in 1867.

In the summer of 1868, a thorough renovation was made with several improvements such as the addition of fifty-four pews, the building extended eastwardly, sanctuary rebuilt, gallery reconstructed, stained glass windows installed from Munich, Germany and the ceiling frescoed and ornamented.

[6] Archbishop James Gibbons consecrated the building on May 1, 1879, assisted by Bishops Thomas A. Becker of Wilmington, Delaware and J. J. Kain of Wheeling, West Virginia.

[7][8][9][10] [11][12] Another renovation, done in 1967, brought the church to the attention of the newly formed Baltimore City Commission on Historical and Architectural Preservation.

In 1965, pastor Thomas J. Donellan formed the Southwest Baltimore Citizens Planning Council, along with four other area churches, to bring identity and renewal to their Hollins Park neighborhood.

The pipe organ, built by Baltimore builder Henry Niemann in 1890, has been placed in storage, pending its location to another church.