Although research has linked insomnia symptoms to repetitive negative thinking (RNT), few studies have examined how insomnia symptoms are associated with RNT over time or specific factors that may account for this relationship. The present study addressed this gap in the literature by examining executive function and emotion regulation as mediators of the relationship between insomnia symptoms and RNT over 3 months. A final sample of 357 unselected community participants completed measures of insomnia symptoms and RNT at time 1, executive function 1 month later (time 2), emotion regulation 2 months later (time 3), and RNT again 3 months later (time 4). Results revealed that insomnia symptoms were associated with increased RNT over 3 months through an indirect effect of executive function on emotion regulation. An alternate model in which emotion regulation at time 2 and executive function at time 3 mediated the effect of insomnia symptoms on RNT was also significant; however, the effect size was relatively reduced. These findings implicate executive function and emotion regulation as factors that may explain the role of insomnia symptoms in the development of RNT observed in many psychiatric disorders.