Of family patrician crémonaise. Studies of right to Pavia and Bologna. Listener (1560), chief clerk and prefect of the Signature (1565), abbot of Nonantula (1566), bishop of Verceil (1572-1587), nuncio in Switzerland (1579-1581), nuncio at the imperial court of Vienna (1581) and first nuncio permanent in Cologne (1584). It entered in relation to Melchior Lussi and accompanied his friend Charles Borromée through the tessinoise part of the archdiocese of Milan (1567), then in the remainder of Switzerland (1570).

At the request of Borromée and cantons of central Switzerland (except Lucerne), Gregoire XIII named it visitor of the bishopric of As in 1578 and nuncio provided with special powers in 1579. He encouraged the creation of the college of the Jesuits of Freiburg (1580) and contributed to make there come Pierre Canisius. He worked for the conclusion of a pact of assistance between the catholic cantons and the bishopric of Basle, and indefatigably visited the parishes and convents of the dioceses of Constancy, Basle, Coire, Lausanne and Sion.

With more or less of success, it gained even the civil authorities with its reforms. But the energy with which it tried to impose the decrees of the Council of Thirty (celibacy, fence) ran up against the resistance of part of the clergy and certain convents (in Argovie and Thurgovie especially), so much so that Rome had to sometimes exhort it with more moderation. In the same way its efforts to draw aside the secular power of attributions of emoluments and the ecclesiastical courts caused the hostility of the governments. The Valaisan Diet and the bishop Hildebrand de Riedmatten were opposed to its visit.

In 1580, it obtained the departure of Beatus has Porta which, bishop of Coire, had refused to reside at it. It projected into 1585 to make deposit Peter Raschèr, elected bishop in 1581 on presentation of the League of House-God. The mission of Bonomi alienated the catholics more and more to him as well as the Protestants; it succeeds in nevertheless anchoring in Switzerland its reform projects, even if their effect remained initially of weak range.