In 316, he was elected bishop of Milan by acclamation, both clergy and people insisting on having him for their Pastor; and reigned until about 328.

[2] Maternus is believed to have discovered at Lodi Vecchio the remains of saints Nabor and Felix, who had been martyred during the reign of Emperor Diocletian in 303, and then relocated these relics to Milan, where a church known as the (Basilica Naboriana) was built in their honor.

He built the chapel of San Vittore in Ciel d'Oro to house the relics of Saint Victor just outside the city walls.

His body was buried in the Basilica Naboriana, that in 1258 was enlarged and dedicated to Saint Francis of Assisi.

On April 14, 1798, shortly before the demolition of the church, his body was moved to the Basilica of Sant'Ambrogio, located a few hundred meters to the south.