To better understand one mechanism by which social cognition affects social outcomes, the current study proposed that young children’s differences in temperament and Theory of Mind (ToM) contribute to teachers’ perceptions of their social competence (SC). Temperament refers to biologically based differences in behavioral regulation and reactivity, whereas ToM describes the process of inferring others’ mental states and making predictions about related behavior. This study examined the effects of ToM on relations between temperament and SC. Moreover, it expanded ToM measurement beyond traditional methods that explicitly provide the information required to correctly ascertain social cognitions (termed truth-based ToM) by introducing a novel approach to defining and measuring ToM that captures the individualistic process of inferring mental states without direct access to all relevant information (termed interpretation-based ToM). Two mediation models were proposed. The first hypothesized that both types of ToM would mediate relations between temperamental effortful control and SC. Results revealed a significant positive indirect effect for truth-based ToM, suggesting that effortful control positively influences truth-based ToM, which in turn positively influences SC. Results did not yield a significant indirect effect for interpretation-based ToM, suggesting that these may be multiply influenced. This was confirmed by the second model which illustrated connections between temperamental negative reactivity, ToM, and SC. It was hypothesized that interpretation-based ToM would mediate relations between negative affectivity and SC, with a moderating effect by effortful control. Results revealed a significant positive moderated indirect effect, suggesting that negative affectivity positively influences interpretation-based ToM, which in turn positively influences SC, specifically when effortful control is high. This study showcased a novel way to define and measure a subtype of ToM that captures the construct more broadly and may be more relevant when interpreting incomplete information than when all situational cues are explicitly provided. Moreover, results of the moderated mediation model illustrated the positive role of negative affectivity when paired with high effortful control in facilitating this more complex form of interpretation-based ToM and eventual SC. Implications of the findings for literature on ToM, temperament, and SC in young children are discussed.