Whilst a good deal of space has been given to discussing the biographical connections between Donald MacKinnon and his most famous pupil, Iris Murdoch, little has been said of the influence of MacKinnon’s own theological writings, particularly Borderlands of Theology (1968) and The Problem of Metaphysics (1974), on the later philosophy of Murdoch. Using the extensive marginalia and notes in her copy of these works stored at the Kingston University archive, this chapter will attempt to outline the impact his thought had on her final work, Metaphysics as Guide to Morals as well as their shared affinities for the work of Kant. It is the contention of the chapter that his lasting impact provided the groundwork for her final work of philosophy.