Returning from a first assignment as a missionary in the Caribbean, Br. Mostyn added graduate studies in theology and spirituality to his ministry as a high school teacher. Completing a doctorate of ministry he devoted his energies to spiritual direction eventually taking a leadership role in developing national and international associations of spiritual directors.

He founded the Office of Mission Integration at Iona College and was later elected to serve as Deputy Congregation Leader of the Congregation of Christian Brothers. Amid the pressures of leadership the art he promoted among his students of Spiritual Direction took on added importance in giving expression to his own interior journey.