Born Andrew Moeslein in Pittsburgh, Pennsylvania on May 7, 1854, he was the son of Caspar Moeslein and Ursula Kirchner. On November 14, 1870 he professed his Passionist vows in Pittsburgh and received the name Mark. He was sent to study at the Pontifical College at Scala Sancta in Rome, Italy. He was ordained in Rome on April 6, 1878.
In 1880 he returned to the United States. He served as a Vicar, Director and Lector. He was an excellent speaker in his missions and retreats. He later served as Lector of Sacred Eloquence in Cincinnati, Ohio and wrote The Mechanism of Discourses. Father Moeslein was an important speaker at the First Passionist Missionary Congress in 1894. He served as Rector of St. Mary’s Monastery, Dunkirk, New York and as Rector of St. Michael’s Monastery, West Hoboken, New Jersey. There he completed the redecoration of the monastery church. He later served as Provincial Consultor to Provincial Stephen Kealy. Father Moeslein had a keen interest in architecture.
In 1913 Bishop Paul Nussbaum, C.P. took possession of the See of Corpus Christi, Texas. This mission territory required yeoman work and Father Moeslein was assigned there to serve the African-Americans at Holy Cross Mission. He built a church, school, and convent. He ministered there fourteen years, when at 73 he was recalled by the provincial, Father Stanislaus Grennan, C.P. In 1927 Bishop Hafey, the first ordinary of Raleigh, North Carolina requested the Passionist assist him in the diocese and Father Moeslein was assigned to the mission which he served until 1943. Again, this was an apostolate primarily to African-Americans. With his health failing he was assigned to St. Joseph’s Monastery, Baltimore, Maryland where he died. His brother Lawrence Moeslein was a Passionist.