Protasewicz was appointed as bishop of Lutsk by Pope Paul III on 27 May 1549 and as bishop of Vilnius by Pope Paul IV on 10 April 1556. At first, he was more interested in political affairs, participating in the Seimas and advising the Grand Duke. Generally, Protasewicz supported judicial independence of the Grand Duchy of Lithuania and opposed a closer union between Poland and Lithuania. In 1568, Protasewicz joined a commission preparing the third Statute of Lithuania and headed it until his death. He was a leading member of the Lithuanian delegations sent to the Polish Great Sejm to negotiate the Union of Lublin.