At the same time, he began studies at the Theological Seminary of the Congregation of the Word of God in Pieniężno.

[3] He was ordained as a Catholic priest on 22 April 1979 in Pieniężno [4] by the archbishop of Przemyśl, Ignacy Tokarczuk.

[2] On 23 March 1998 Pope John Paul II appointed him auxiliary bishop of the apostolic administration of Siberia with the titular see of Tabunia.

[1] Mazur choose the phrase "Veni, Sancte Spiritus" (Come, Holy Spirit) as his bishop's motto.

[8] Thanks to his involvement in talks with state authorities and representatives of the Orthodox Church, it was possible to consecrate the new Immaculate Heart of Mary Cathedral in Irkutsk in 2000.

In April 2002, he was arrested at the airport in Moscow on his return trip to Irkutsk and forcibly sent back to Poland.

[2] On 17 April 2003, Pope John Paul II installed Mazur as the new bishop of Ełk, succeeding the deceased Edward Samsel.