A growing body of research suggests that mindfulness-based interventions may be a valuable method for reducing internalizing symptoms in autistic individuals. The current study extends this work using an effectiveness-implementation hybrid type 1 study. In this parallel randomized controlled trial, we examined a novel telehealth intervention for autistic adolescents and their caregivers. MINDful TIME includes eight weekly group meetings and regular use of a commercially available mindfulness meditation app. Participants were 42 adolescent-parent dyads randomized to the treatment or delayed treatment control (DTC) group. The program was implemented with fidelity (91.24%-94.78%), and 90% of treatment group dyads completed the program. On average, participants attended >90% of sessions and reported high acceptability. Treatment group adolescents demonstrated statistically and clinically significant reductions in parent-reported depression symptoms relative to DTC (F(1, 34) = 7.31, p = 0.01, n[subscript p superscript 2] = 0.18). Female adolescents in the treatment group showed significant reductions in parent-reported anxiety symptoms (F(1, 34) = 4.22, p = 0.05, n[subscript p superscript 2] = 0.11). Exploratory analyses indicated treatment-related improvements in adolescent executive functioning, parent mindfulness and well-being, and adolescent-parent relationship dysfunction. Findings warrant future examination of MINDful TIME to address well-documented challenges with mental health in this population. This trial was registered on clinicaltrials.gov (NCT05685589).