Johnston's academic journey began with a BA in Philosophy from Westmont College, followed by an MDiv from Gordon-Conwell Theological Seminary and a PhD in Islamic Studies from Fuller Theological Seminary. He has served in various academic capacities, including teaching in North Africa and the Middle East. His research focuses on the intersection of law and theology in modern Islamic scholarship, with notable publications such as "Earth, Empire and Sacred Text" and "Muslims and Christians Debate Justice and Love." Currently, Johnston holds positions as an adjunct lecturer at St. Joseph's University and Affiliate Assistant Professor of Islamic Studies at Fuller, where he continues to contribute to scholarly discourse in his field.