Importance: Monitoring one's performance is necessary for learning and adaptive behavior; however, heightened performance monitoring is a purported endophenotype of obsessive-compulsive disorder (OCD). The anterior cingulate cortex (ACC), a brain region implicated in the pathogenesis of OCD, is associated with performance monitoring. Whether performance monitoring early in development is an identifiable risk factor for OCD and whether early childhood performance monitoring is associated with later alterations in ACC volume are unknown. Objective: To determine whether an observed indicator of heightened performance monitoring during the preschool age is associated with later onset of OCD and altered dorsal ACC (dACC) volume through adolescence. Design, Setting, and Participants: This longitudinal observational cohort studywas performed at an academic medical center as part of the Preschool Depression Study. A sample of 292 children oversampled for depression from September 22, 2003, through May 12, 2005, completed a performance-based observational task during which they received persistent negative evaluation. Blind raters behaviorally coded child performance monitoring. During the next 12 years, children completed annual diagnostic assessments; 133 completed the final behavioral follow-up and 152 contributed 1 to 3 magnetic resonance imaging scans. Follow-up was completed on August 14, 2017. Main Outcomes and Measures: Onset of DSM-5 diagnosis of OCD from baseline to the final behavioral assessment and whole-brain-adjusted dACC volume at the 3 waves of scanning. Results: Among the 292 preschool children who completed the baseline evaluation (51.4% boys; mean [SD] age, 4.5 [0.8] years), when controlling for demographic and clinical indicators, those who exhibited observed heightened performance monitoring were 2 times more likely to develop OCD (n = 35) during the next 12 years (odds ratio, 2.00; 95% CI, 1.06-3.78; P = .03). Multilevel modeling of dACC volume across the 3 scan waves (n = 152) demonstrated that heightened performance monitoring was associated with smaller right dACC volume (intercept estimate, -0.14; SE, 0.07; t = -2.17; P = .03). Conclusions and Relevance: An ecologically valid indicator of performance monitoring in early childhood was associated with onset of OCD and smaller dACC volumes in later childhood and adolescence. Early childhood observed performance monitoring is a readily observed risk factor of OCD that can be identified in preschool-aged children.