The present-day sacristy of San Carlino is composed of two rooms corresponding to the former ante-refectory (also called Sala De Profundis) and refectory of the Trinitarian community, designed by Francesco Borromini and which served their purpose until the enlargement of the convent in the 18th century by Alessandro Sperone.

Borromini designed the refectory of San Carlino with smooth walls, well lit by three large windows and roofed with a low vault and lunettes. Beautiful stucco mouldings run along the cornice and the edges of the lunettes. In each of the four corners of the room, under the corresponding vaulting, the winged face of a cherub, so characteristic of the Borrominian style, appears in relief.

After the extension of the building in the 18th century, the refectory was converted into a sacristy; the pew and tables were removed and beautiful chestnut furniture was installed, with large chests of drawers to house the various liturgical vestments.

The present sacristy houses the painting of 'St. Charles Borromeo in adoration of the Trinity' by Orazio Borgianni. This oil painting, which presided over the altar, is the only surviving evidence of the original church, which existed from the foundation of the community in 1612 until the work of Francesco Borromini in 1634.