The present study examined whether improvements in insomnia affect symptoms of worry, anxiety, depression, and impaired social functioning after transdiagnostic treatment for excessive worry. Forty-nine participants with clinical excessive worry were randomized to either a behavioral activation for worry (BAW) treatment group or a waitlist control group. BAW treatment was administered over the course of eight weekly group-based treatment sessions that included the following components: psychoeducation, self-monitoring, functional assessments of worry, activity scheduling, and relapse prevention. After treatment, 64% of patients in the BAW treatment group no longer met the criterion for insomnia, as compared to 17% of patients in the control group. In addition, the BAW treatment group had significantly smaller changes in Insomnia Severity Index (ISI) scores from pre-treatment to post-treatment (ΔISI) compared to the control group. Furthermore, ΔISI scores significantly affected changes in anxiety, depression, and impaired social functioning scores, but had no effect on the pathological worry score. Our results showed that insomnia had a full mediation effect on anxiety and impaired social functioning, and a partial mediation effect on depression. Despite the small sample size, our findings suggest that the amelioration of insomnia may decrease anxiety, depressive symptoms, and social dysfunction in patients with anxiety or depression.