Background: This study examined whether social-cognitive processes in children mediate relations between mothers' depressive symptoms across the first 3 years and children's first-grade social competence. Three maladaptive cognitions were examined: self-perceived social inadequacy, hostile attribution, and aggressive response generation. Method: One thousand three hundred and sixty-four mothers reported depressive symptoms across early development, first-grade children reported target social cognitions, and children's first-grade social competence was observed and reported by multiple informants. Results: Findings demonstrated that (a) mothers' average depressive symptoms from 6 to 36 months predicted children's maladaptive social cognition in first grade, (b) low mother-child responsiveness mediated this relation, and (c) maladaptive social cognition mediated relations between mothers' early depressive symptoms and low first-grade social competence independent of later depressive symptoms. Conclusion: When mothers' depressive symptoms occur early in development, they may set in motion low-responsive dyadic patterns that promote children's maladaptive social cognition and, as a result, low social competence.