Longitudinal trajectories of psychopathology in previously institutionalized (PI) youth were identified and biobehavioral emotion regulation processes were examined as developmental mechanisms that predict these trajectories. Mental health data were collected from PI (N = 132) and nonadopted (NA; N = 175) youth across four time points (participant age ranged from 7- to 21-year-old). Using semiparametric group-based methods, the probability that each individual belonged to a distinct group that followed a specific pattern of behavior across time was estimated. We then tested whether unique aspects of emotion regulation (global, observed, and biological) were differentially associated with membership in externalizing and internalizing trajectory groups using multinomial logistic regression models. Four externalizing trajectories were identified for the PI and NA groups. For PI youth, global, observed, and biological emotion regulation processes were uniquely predictive of more adaptive externalizing trajectories. For NA youth, only parent-reported global emotion regulation was predictive of externalizing patterns. Three internalizing trajectories were identified for PI and NA youth. Generally, only parent-reported global emotion regulation predicted internalizing group membership for both PI and NA youth. Results suggest that biobehavioral emotion regulation processes may be particularly important predictors and potential points of intervention when targeting trajectories of externalizing behaviors in PI children.