According to tradition, the church was dedicated to St. Mark, patron of Venice, after the help given by that city in the war against Frederick Barbarossa in the 12th century. However, the first mention of the church dates from 1254 when the Augustinians built a Gothic style edifice with a nave and two aisles re-using pre-existing constructions.
The structure was heavily modified in the Baroque style during the 17th century, when it became the largest church in the city after the Duomo di Milano.
In early 1770, the young Mozart resided in the monastery of San Marco for three months and, on May 22, 1874, the first anniversary of the death of the Milanese poet and novelist Alessandro Manzoni was commemorated in the church by the first performance of Verdi's Requiem, written in his honour.
The façade dates from an 1871 restoration by Carlo Maciachini, who kept the marble portal with tympanum, a gallery of small arches, the rose window and three statues of saints attributed to Giovanni di Balduccio or the Master of Viboldone. In the lunette is a mosaic representing the Madonna between Saints, a copy of the original by Angelo Inganni.
The campanile dates from the 14th century. It was restored and completed in 1885. The interior, in the Baroque style, has a nave and two aisles.
In the first chapel on the right are frescoes by Gian Paolo Lomazzo. In the right transept is a fresco by the Fiammenghini with Alexander IV Instituting the Order of the Augustinians, under which a 14th-century Crucifixion was discovered in 1956. The author of the latter has been identified by Anovelo da Imbonate. The right arms of the transept houses also several sarcophagi from the mid-14th century, including the tomb of Lanfranco Settalo, counsellor of Archbishop Giovanni Visconti, by Giovanni di Balduccio.
Near the rear exit is a 16th-century tombstone portraying the Angel of the Resurrection, another fresco by the Fiammenghini (under which is a 14th-century fresco). On the side walls of the presbytery are the Dispute of St. Ambrose and St. Augustine by Camillo Procaccini and the Baptism of St. Augustine by Giovanni Battista Crespi.
References:The Church of St Donatus name refers to Donatus of Zadar, who began construction on this church in the 9th century and ended it on the northeastern part of the Roman forum. It is the largest Pre-Romanesque building in Croatia.
The beginning of the building of the church was placed to the second half of the 8th century, and it is supposed to have been completed in the 9th century. The Zadar bishop and diplomat Donat (8th and 9th centuries) is credited with the building of the church. He led the representations of the Dalmatian cities to Constantinople and Charles the Great, which is why this church bears slight resemblance to Charlemagne's court chapels, especially the one in Aachen, and also to the Basilica of San Vitale in Ravenna. It belongs to the Pre-Romanesque architectural period.
The circular church, formerly domed, is 27 m high and is characterised by simplicity and technical primitivism.