Purpose: Early language difficulties are associated with later internalizing problems across different ages and for different aspects of language. The mechanisms behind this association are, however, less understood. In the current study, we investigated longitudinal associations between language difficulties at 5 years and internalizing problems at 6 years. We also examined emotion regulation, empathy, assertiveness, and social engagement at 6 years as possible pathways for this association. Method: A subsample from the Norwegian Mother, Father and Child Cohort Study (MoBa) was used (N = 928). Structural equation models were developed to test the longitudinal associations and indirect pathways between language and internalizing problems. Results: The results showed high stability for internalizing problems from 5 to 6 years ([beta] = 0.59, p < 0.001). Furthermore, semantic language difficulties predicted change in internalizing problems ([beta] = 0.12, p < 0.001). Finally, the path between semantic language and internalizing problems was partially mediated by social engagement and emotion regulation, with the indirect pathways accounting for 55% of the initial association. For girls, there was a significantly stronger correlation (p < 0.05) between semantic language difficulties and internalizing problems at baseline (r = 0.30, p < 0.001) than for boys (r = 0.16, p < 0.001). Otherwise, there were no sex differences. Conclusions: Indirect pathways from language difficulties to internalizing problems were identified through social engagement and emotion regulation. The results may guide targets for intervention in groups of children with language difficulties at risk for developing internalizing problems.