Background: Family socio-economic position (SEP) is known to be associated with adolescent mental health. Whether the relationship is different for different mental health dimensions is unknown. Methods: Using a cross-sectional design, we investigated the differential effects of family SEP on multiple mental health dimensions in preadolescents (N = 2230, baseline age 10–12, 49% boys) using reports from multiple informants (parent, self, and teachers). A score equal to or higher than the 85th percentile (averaged across informants) defined mental health problems. Results: SEP was inversely associated with all dimensions. Compared to high SEP, the odds ratios (OR) for externalizing problems were 3.88 (95% confidence interval (CI): 2.56, 5.90) and 2.05 (CI: 1.34, 3.14) for low and intermediate SEP, respectively. For internalizing problems, they were 1.86 (CI: 1.28, 2.70) and 1.37 (CI: 0.94, 2.00), respectively. When adjusted for externalizing problems, SEP effects on internalizing problems materially attenuated (OR: 1.47, CI: 0.78, 1.68 and OR: 1.34, CI: 0.91, 1.96) while the converse was less pronounced (OR: 3.39, CI: 2.24, 5.15) and (OR: 1.91, CI: 1.25, 2.94). Conclusion: In early adolescence, the risk of mental health problems increases with decreasing SEP, particularly for externalizing problems. Further, the SEP-internalizing problems relationship is partly explained by shared aspects with externalizing problems.