Fr Themann was born in Cincinnati, Ohio, and attended the University of Cincinnati as a young man. After receiving a Bachelors of Science in Industrial Management, he entered St Thomas Aquinas Seminary, located in Winona, Minnesota, at the time. After six years of formation there, he was ordained a priest in June of 2009. Fr Themann's first assignment was at the busy parish and school of the SSPX in Saint Marys, Kansas. During his time there, he was chaplain of the college and serviced the SSPX's mission chapel in Oklahoma City, Oklahoma. Three years of apostolic labours in Saint Marys were followed by three years as a seminary professor at St Thomas Aquinas Seminary, from which, in addition to his professorial duties, he serviced the SSPX's mission chapel in Green Bay, Wisconsin. In 2015, Fr Themann was nominated as rector of Holy Cross Seminary, taking up his duties in August of that year.
Besides the many cares of administration involved in the post of rector, Fr Themann is prefect of the sacristy department, and teaches the classes of Ascetical & Mystical Theology, Introduction to Scripture, the Seminary Rule. He delivers a spiritual conference to the seminarians five evenings of the week.