Until recently, the oldest evidence of the existence of the church of San Cassiano was the thirteenth-century Liber Notitiae Sanctorum Mediolani attributed to Goffredo da Bussero. In fact, recent restoration works have brought to light more ancient remains, in particular a preparatory drawing for a fresco with men and animals, datable to the fifth or sixth century.

The building underwent modifications and extensions over the course of its history. In 1836, renovation works were carried out which brought it to its present appearance; in particular, the orientation was changed: from east-west and north-south.

The salient façade is the result of a reconstruction in 1932; from it we can deduce the internal division into three naves.

Along the sides of the church, two bodies protrude from the main structure: they are the two side chapels dedicated to the Madonna and San Giuseppe; in the rear area of ​​the one on the left stands the bell tower.

The interior is divided into three naves, with two side chapels and the semicircular presbytery flanked on the left by a room used as a chapel.

The side altar of the right aisle is adorned with a sixteenth-century fresco of the Madonna with Child (also called Madonna di San Cassiano), torn from the main altar and placed here following the nineteenth-century renovation works. The chapel that houses it is decorated on the walls with frescoes by Luigi Sabatelli: the Expulsion of Adam and Eve and God announcing the coming of the Madonna who will crush the head of the snake.

Continuing on, in the presbytery area there are other paintings, the work of Romeo Rivetta from 1910; on the side walls we recognize a Madonna and San Carlo Borromeo administering the Eucharist to the sick, while on the vault the Glory of the Eucharist and the Evangelists.

In the lower band of the wall between the presbytery and the main nave is the aforementioned preparatory drawing with men and animals, brought to light during recent restorations, surmounted by fragments of a decorative band.

In the room adjacent to the presbytery there are preserved, in addition to processional banners, two seventeenth-century paintings depicting San Pietro and San Domenico.

Proceeding in the left aisle towards the counter-façade you will find the side chapel, with frescoes by the painter Cisterna from 1921 (Marriage of the Virgin and Death of St. Joseph), and the baptistery: the baptismal font is covered by a carved wooden structure dating back to the 18th century .

In the main nave, the fresco depicting the Glory of San Cassiano can be seen on the vault and the remains of tombs brought to light during the aforementioned restorations on the floor.