Theory of mind has received intensive attention in research as an important skill to develop. Furthermore, recent evidence indicates its role in discourse comprehension. In the current study, we examined the mediating role of theory of mind in the relations of foundational language and cognitive skills (working memory, attentional control, vocabulary, and grammatical knowledge) to discourse comprehension using the direct and indirect effects model of text comprehension and production (Kim, 2016) as a theoretical framework, and using longitudinal data from kindergarten to Grade 2. Structural equation model results showed that theory of mind partially mediated the relations in both grades, and the effects (standardized regression weights) were similar in kindergarten and Grade 2. Interestingly, the relations of language and cognitive skills to theory of mind differed in kindergarten versus Grade 2. Language and cognitive skills had moderate to strong longitudinal stability, and these skills in kindergarten were indirectly related to discourse comprehension in Grade 2 via the language and cognitive skills in Grade 2. These results support the mediating role of theory of mind as well as the nature of structural and longitudinal relations among language and cognitive skills and to discourse comprehension.