This is the first full-length critical study of William of Nassington and his 16'396-line poem in rhymed couplets, the Speculum Vitae. Nassington is identified in a family of prominent ecclesiastics from medieval York. This study establishes Nassington as the poet of the Speculum Vitae and the Tractatus de Trinitate et Unitate, and it resolves critical errors initiated by Warton and Horstmann regarding Nassington’s identity, canon, and sources. Circulation of nearly forty extant manuscripts of the Speculum Vitae confirm its popularity as a compendium of orthodox doctrine and as a guide to mystical prayer. An edition of Ms. Royal C. viii, prepared by John Smeltz, established a text which is described in this study and used to illustrate Nassington’s theology. Nassington draws from the patristic sources and follows literary traditions of the summa, speculum and Pater Noster. The Speculum Vitae is schematized as a pentad of seven petitions of the Pater Noster, gifts of the Holy Spirit, Vices, Virtues, and Beatitudes. Three expositions of the Pater Noster petitions correspond to the stages of purgation, illumination, and mystical union. However, Nassington’s emphasis is on mystical prayer, which he describes from his authority and experience. Nassington’s work is a speculum of fourteenth-century religious England.