Bishop Mroziewski was ordained to the priesthood on June 29, 1991, in Poland, and was assigned to the Diocese of Brooklyn in 1992.

Then-Father Mroziewski was assigned to Our Lady of Czestochowa–St. Casimir as a parochial vicar in 1993 and began his studies of Canon Law and administration at the Catholic University in Lublin in 1997.

Upon his return to the Brooklyn Diocese, Father Mroziewski was appointed as the administrator of Our Lady of Czestochowa–St. Casimir Parish, and in 2002, was named pastor.

In 2013, Father Mroziewski was assigned to his current position as pastor at Holy Cross Church. On July 20, 2015, Father Witold Mroziewski was ordained an Auxiliary Bishop of the Diocese of Brooklyn.

Bishop Mroziewski holds a Doctorate in Canon Law and has served in the Diocese of Brooklyn Tribunal as an attorney and judge, working on marriage-related cases.

He serves as the vicar for Migrant and Ethnic Apostolates for the Brooklyn Diocese and serves on the USCCB Subcommittee on Asian and Pacific Island Affairs, and on the Aid to the Church in Central and Eastern Europe Subcommittee.

Bishop Mroziewski was named the episcopal vicar for Queens in March 2022.