Socially withdrawn children are more likely to be heterosexually inactive in early adulthood. This study aimed to test a three-mediator model. We hypothesized that social withdrawal during childhood would hinder a sense of social competence in early adolescence, limiting other-gender friendships in mid-adolescence, in turn limiting involvement in romantic relationships in late adolescence, and thus favoring a later transition into sexual activity. This mediation model was tested on a sample of 332 participants assessed annually between the ages of 12 and 22. Structural equation modeling revealed a significant indirect effect of social withdrawal on late transition into heterosexual activity through the three tested mediators. The direct effect of social withdrawal on late transition into heterosexual activity was also significant, suggesting a partial mediation.