This thesis provides a detailed analysis of the identifiable aspects of Mary Tudor's personal religious beliefs and practices from childhood to the end of her reign. It carefully examines a variety of evidence including translations done by Mary, book and manuscript dedications written by and to Mary, correspondence, first-hand accounts, Mary's privy purse expenses, her will, and Marian acts and proclamations. This analysis puts Mary's personal religiosity in conversation with her religious policy as queen. Mary demonstrated pious dedication to the Mass and other traditionally Catholic liturgical elements and nearly unwavering dedication to Rome and the papacy. During her reign Mary and her government promoted a religious policy that was consistent with core aspects of Mary's personal religiosity. This thesis concludes that Mary was not only theologically Catholic and dedicated to the traditional liturgical rites, like her father Henry VIII, but also dedicated to England's ecclesiastical relationship with Rome.