[2] In 1843, Mullen accepted an invitation from Bishop Michael O'Connor to immigrate to the United States and join the Diocese of Pittsburgh.

[2] After completing his theological studies, he was ordained to the priesthood for the Diocese of Pittsburgh by Bishop O'Connor on September 1, 1844.

[3] He served for about two years as curate at the Cathedral of Pittsburgh, and was afterwards charged with the care of congregations at Johnstown and in Jefferson County.

[1] He was transferred to the rectorship of St. Peter's Church at Allegheny in 1854, and served as vicar general of the Diocese of Pittsburgh from 1864 to 1868.

[3] During his 31-year-long administration many priests were ordained, parishes established, churches and schools built, and conferences for the clergy held.