In 1520, Mauritius and his brother Eberhard had to leave Danzig due to political conflicts of Eberhard. He sided with the King of Poland who nominated him in 1523 to the vacant seat of Prince-Bishop of Warmia. He was elected and started his administration in the fall of 1523 before being consecrated on 6 December. The diocese had suffered both by the administrator of his predecessor and by the Polish–Teutonic War (1519–1521). During peace talks in Cracow in 1525, he managed to defend the except bishopric against demands of both the Polish king and the former Teutonic Grandmaster who became the first Duke of Prussia. Until 1531, Ferber managed to improve the economic situation in his diocese as well as preventing towns from becoming Lutheran, with Elbląg (Elbing) being the exception to this rule.