Brain stimulation shows promise as an intervention to enhance executive function, particularly when paired with cognitive training. To optimize such approaches, we must understand the potential role of individual differences in intervention outcomes. We investigated the combined effects of multi-session multitasking training and prefrontal transcranial direct current stimulation (tDCS) on generalization of performance benefits, focusing on how cortical morphology predicts performance improvements. One hundred seventy-eight individuals underwent 7 Tesla MRI before completing multisession training with online stimulation. A cognitive task battery assessed improvements in trained and untrained tasks pre- and post-training. Stimulating the left or right prefrontal cortex at 1 mA during multitasking training enhanced transfer to a visual search task. Critically, cortical morphology predicted stimulation efficacy for inducing transfer. Cortical thickness in regions beneath the stimulating anode was related to reaction time changes in the most difficult visual search condition but only for the left and right 1 mA multitasking training groups. Performance was not related to cortical thickness for the groups receiving sham stimulation, 2 mA stimulation, or 1 mA stimulation with a control training task. These results highlight the importance of individual anatomical differences in modulating tDCS efficacy and identifying specific neuroanatomical features that predict generalized performance gains from combining tDCS with cognitive training.