Purpose: Active engagement in one's therapy is a key contributor to successful outcomes. Research on child engagement in cognitive behaviour therapy (CBT) has largely focused on youth without autism. This longitudinal study examined multiple indicators of child engagement in relation to outcomes for autistic children who took part in CBT for emotion regulation. Method: Data were collected from 60 autistic children who were between 8 and 13 years of age (86.7% male; M[subscript age] = 9.58 years, SD = 1.44 years; 75% White). Indicators of child engagement included independent observer ratings of in-session involvement, as measured by the "Child Involvement Rating Scale", and therapist ratings of the therapeutic relationship and homework completion using single-item measures. Indicators of engagement were measured at early (i.e., first third), middle (i.e., mid third), and late (i.e., final third) stages of treatment. Parent-reported emotion regulation was the primary treatment outcome, as measured by the "Emotion Regulation Checklist." Results: After controlling for pre-treatment scores, in-session involvement significantly predicted some aspects of post-treatment emotion regulation, whereas therapeutic relationship and homework completion did not. Conclusions: Child in-session involvement throughout therapy may be particularly relevant for treatment change. Addressing issues related to in-session involvement early in treatment may help to promote therapeutic success for autistic children.