Almost nothing is known about the life and the episcopate of Magnus. Magnus lived under the Arian king Theodoric the Great, who probably at first supported him, but later persecuted him as already happened for the philosopher Boethius.
What is surely known, is the text of his funeral epitaph, transmitted us by Goffredo da Bussero (13th century), which describes Magnus as a men of great charity who helped the prisoners of war.
Magnus died in 530, on the 1 December. His remains were interred in the Basilica of Sant'Eustorgio in Milan. A late tradition, with no historical basis, associates Magnus with the Milanese family of the Trincheri.
The first formal survey on his relics was made in 1248 by the Domenicans who ministered to the Basilica of Sant'Eustorgio in Milan. The main church of the town of Legnano, about 20 km (12 mi) from Milan, is dedicated to Magnus. The Basilica di San Magno of Legnano was built between 1503 and 1513 and a part of relics of Magnus were translated there on 5 November 1900. His feast is celebrated on November 5 in such basilica and together all the saint bishops of Milan on 25 September.