Trinity Church was the first major work of Henry Hobson Richardson, and the building established the “Richardsonian Romanesque” architectural style, which strongly influenced public buildings of the late 19th century. Richardson was inspired by the European churches he saw that were built in the early medieval period called Romanesque, and also by Byzantine architecture.

The building was created by four men in their 30s—the rector, Phillips Brooks; the architect, H. H. Richardson; the master builder, O. W. Norcross; and the mural painter and stained glass artist, John La Farge.

The floor plan of the church approximates a Greek Cross, with near equal legs projecting out from the center which sits below the grand tower. The goals were to provide a feeling of openness, and to ensure that everyone could hear and see the preacher, without having their view obstructed by columns.

The property on which Trinity stands—along with the rest of the Back Bay area—is landfill. To keep the stout granite and sandstone building that anchors Copley Square from sinking below the high water table, the church foundation is supported by thousands of vertical wooden pilings. The property on which Trinity stands—along with the rest of the Back Bay area—is landfill. To keep the stout granite and sandstone building that anchors Copley Square from sinking below the high water table, the church foundation is supported by thousands of vertical wooden pilings.

The West Porch, the main entrance off of Copley Square, was added to the church nearly 20 years after the building was completed by Shepley, Rutan & Coolidge. John Evans, Hugh Cairns, and M. Domingo Mora sculpted the frieze that features great leaders in Christian history, and scenes from the life of Jesus. 

More than 21,500 square feet of the interior is covered with painted decoration and murals, completed. Nearly all of it was executed in only five months over the winter of 1876-1877, before there was heat in the building. John La Farge was assisted with the figural and ornamental work by a team of six-to-eight artists, including Augustus Saint-Gaudens and Francis Millet who would become known in their own right. All their names are included over east tower arch. 

The décor and design of the chancel has changed over the years. Its most recent incarnation, featuring marble and gilded decoration and a dramatically decorated altar, was completed by the firm Maginnis & Walsh in the 1930s.

There are many sculptures of Phillips Brooks in and around Trinity Church, the most prominent being the free-standing monument on the north side of the church facing Boylston Street. Designed by Augustus Saint Gaudens, the statue represents Brooks preaching under the inspirational hand of Jesus. (1910)