Four plain columns topped with composite capitals mark the ends of the facade and the confluence of the two lateral concavities and the central convexity of the facade.

A horizontal frieze between the columns divides this lower level into two further sub-levels: in the lower central section is the entrance door to the church, set on three steps and with columns next to the jambs supporting its lintel; in each of the side sections is an oval window bordered in stone with vegetal motifs, a sculpted stag's head at the bottom and a crown at the top.

Above the entrance, two cherubs flank the figure of Saint Charles Borromeo, a sculpture made by Antonio Raggi in 1675. On either side are sculptures of St. John of Matha and St. Felix of Valois, founders of the Trinitarian Order, sculpted by Sillano Sillani in 1682.

The two levels of the façade are divided by a large frieze that runs from one end to the other and contains the following inscription: "In Honorem SS. Trinitatis Et D. Caroli MDCLXVII" (In honour of the Holy Trinity and Saint Charles 1667).

After Francesco Borromini's tragic suicide, his nephew Bernardo completed the upper level of the facade of San Carlino, maintaining the same architectural style as his uncle.

To this end, he superimposed the four columns described above with another four, but shorter and narrower ones. He also placed a glazed door in the centre of a stone edicule crowned with a pinnacle to illuminate the interior of the church and provide access to an exterior balcony topped by a balustrade similar to that of the cloister.

The facade is crowned by a large oval supported by two angels that contained a fresco that was lost - representing the Virgin Mary crowned by the Holy Trinity - as well as a balustrade similar to the previous ones and adapted to the sinuous lines of the facade, which runs from one end to the other.

Bernardo rebuilt the bell tower located on the corner of the crossroads and over one of its four artistic fountains. This bell tower has a quadrangular structure with large lateral openings covered with pilasters in the form of half-columns.

It is topped by a pinnacle and crowned with a bronze ball and a cross. Two of the bells, dated 1658, protrude from two of its openings.