Henry Manning’s (1808–92) transition from Anglican to Roman Catholic convert has not received the extensive attention that John Henry Newman’s journey to Roman Catholicism has received. Though more than a half dozen treatments have appeared in recent decades, newly acquired archival resources received by the Westminster Diocesan Archives in 2014 warrant a new appraisal of the events leading to his conversion. How could a committed adherent of the Oxford Movement, who did not initially follow Newman’s example in 1845, make the decision to leave the Church of his birth in 1851? What interior process enabled Archdeacon Henry Manning to preside over the assembly of Chichester clergy that condemned ‘papal aggression’ in 1850, and announce at the conclusion of the vote that he would be received into the Roman communion? This article outlines undercurrents in Manning’s thought, traces of which can be found in his undergraduate years, and considers concepts that culminated in the decision that changed his life, and guided his Roman Catholic ecclesial outlook. His role in shaping the agenda of Vatican I and the post-conciliar era heightens the significance of this background.