Developmental delays in cognitive flexibility early in elementary school can potentially increase vulnerability for subsequent externalizing and internalizing psychopathology. The first goal of the current study was to identify latent subgroups of children characterized by different developmental trajectories of cognitive flexibility throughout kindergarten and first grade using data from the Early Childhood Longitudinal Study, Kindergarten Class of 2010-2011 dataset. The second goal was to examine whether identified longitudinal developmental trajectories of cognitive flexibility could be associated with internalizing and externalizing behaviors in the second grade, while accounting for background child (age, gender, and Spanish-speaking) and family (family income and mother's education) covariates. The analytic sample consisted of 15,827 kindergarteners (51.20% male; 48.50% White, 13.5% Black/African American, 24.3% Hispanic/Latino, 7.60% Asian, and 6.1% other), who were approximately 5.62 years old (SD = 4.48 months) at the study's outset. Most children lived in households with medium family income of approximately $50,000-$55,000. Using a growth mixture modeling approach, our analyses identified normative (91.05%; 50.4% male) and delayed (8.95%; 59.4% male) cognitive flexibility groups and demonstrated that delayed developers have higher levels of externalizing and internalizing behaviors in the second grade, even after adjusting for background covariates. Our findings, in conjunction with research on cognitive flexibility training, suggest that caregivers may lower the risk for externalizing and internalizing behaviors in delayed developers by correcting inflexible thinking, encouraging alternative solutions, and providing emotional support when children face challenging problems. (PsycInfo Database Record (c) 2023 APA, all rights reserved).