From 1551 he was the Prince-Bishop of the Bishopric of Warmia in Royal Prussia, and from 1558, he served as the papal legate to the Holy Roman Emperor's Imperial Court in Vienna, Austria.

Piotr Tomicki, Bishop of Kraków and Vice-Chancellor of Poland, employed him as private secretary and entrusted to him the education of his nephews.

[1] After graduating as doctor of canon and civil law at the University of Bologna on 8 June 1534, he returned to Krakow and became secretary in the royal chancery.

Hosius was then sent by Sigismund on a diplomatic important mission to the courts of King Ferdinand I at Prague and Emperor Charles V at Brussels and Ghent.

Hosius and Marcin Kromer were the two bishops most instrumental in keeping the Warmia region Catholic, and neighbouring Ducal Prussia became Protestant.

[3] The following year, Pope Pius IV appointed Hosius as his personal nuncio to Ferdinand I, Holy Roman Emperor, at the court in Vienna, where he was to work on the reopening of the Council of Trent.

Pope Pius IV named him Legate-Theologian for the third session of the Council of Trent; the other two legates were Cardinals Puteo and Gonzaga.

When the Council ended, he returned home, despite requests to travel to Rome for the papal conclave that was to be held after the death of the ailing Pius IV.

Both Kromer and Hosius left many records of their German speeches and sermons in their years of duty in the Bishopric of Warmia.