Although inhibitory control is typically associated with positive outcomes, several theoretical frameworks suggest that too little "and" too much inhibitory control may be problematic. Using a longitudinal, latent variable approach, we examined whether a multi-method index of inhibitory control at Time 1 (N = 105, 52 girls, M[subscript age] = 3.50 years, 87% White) predicted observed social behavior with an unfamiliar peer and maternal report of preschoolers' mental health difficulties at Time 2 (M[subscript age] = 4.76 years). Data collection occurred between 2017 and 2019. Inhibitory control displayed a U-shaped relation with prospective outcomes, where high "and" low levels of inhibitory control were associated with higher levels of avoidant social behaviors and mental health difficulties. The results are discussed in the context of under- and over-regulation in understanding individual differences in children's social behavior and mental health difficulties.