The human brain is organized into large-scale functional modules that have been shown to evolve in childhood and adolescence. However, it remains unknown whether the underlying white matter architecture is similarly refined during development, potentially allowing for improvements in executive function. In a sample of 882 participants (ages 8–22) who underwent diffusion imaging as part of the Philadelphia Neurodevelopmental Cohort, we demonstrate that structural network modules become more segregated with age, with weaker connections between modules and stronger connections within modules. Evolving modular topology facilitates global network efficiency and is driven by age-related strengthening of hub edges present both within and between modules. Critically, both modular segregation and network efficiency are associated with enhanced executive performance and mediate the improvement of executive functioning with age. Together, results delineate a process of structural network maturation that supports executive function in youth. Display omitted •Structural brain modules become more segregated during youth•Targeted strengthening of hub edges simultaneously promotes network efficiency•Enhanced modular segregation mediates improvements in executive function in youth Baum et al. apply network analytic techniques to demonstrate that human white matter networks become increasingly modular during adolescent development. Furthermore, targeted strengthening of hub connections facilitates global network integration. This process of network evolution mediates improvements in executive functioning during youth.