After graduating from the University of Pennsylvania, he received his Ph.D. at the University of Chicago and then conducted postdoctoral work at Yale University. His work focuses on integrating phylogenetic analysis, data from the fossil record, and insights from ecology and functional morphology in order to understand the drivers of morphological diversification in plant reproductive structures. Most of Leslie’s work has focused on conifers, a group with considerable living diversity, diverse reproductive morphologies, an extensive fossil record stretching back more than 300 million years, and a well-resolved phylogeny. His research also includes field and specimen-based paleobotany, including ongoing projects in the Jurassic of Argentina, the Cretaceous of Mongolia, and the Miocene of New Caledonia.