A large literature assessing personality across the lifespan has used the Big Five as an organizing framework, with evidence that variation along different dimensions predicts aspects of psychopathology. Parent reports indicate that these dimensions emerge as early as preschool, but there is a need for objective, observational measures of personality in young children, as parent report can be confounded by the parents’ own personality and psychopathology. The current study observationally coded personality dimensions in a clinically enriched sample of preschoolers. A heterogeneous group of preschoolers oversampled for depression (N = 299) completed 1–8 structured observational tasks with an experimenter. Big Five personality dimensions of extraversion, agreeableness, conscientiousness, neuroticism, and openness to experience were coded using a “thin slice” technique with 7,820 unique ratings available for analysis. Thin slice ratings of personality dimensions were reliably observed in preschoolers ages 3–6 years. Within and across-task, consistency was also evident, with consistency estimates higher than found in adult samples. Divergent validity was limited, with coders distinguishing between three (extraversion/openness; agreeableness/conscientiousness; and neuroticism) rather than five dimensions. Personality dimensions can be observationally identified in preschool-age children and offer reliable estimates that stand across different observational tasks. Study findings highlight the importance of observational approaches to assessing early personality dimensions, as well as the utility of the thin slice approach for meaningful secondary data analysis.