Autistic adults are at a greater risk for mental health problems compared to the general population, with 50% meeting criteria for a co-occurring psychiatric condition. Depression and anxiety are the most common of these conditions among autistic adults, contributing to long-term detrimental effects on health, day-to-day functioning, and quality of life. This study will conduct the first large-scale head-to-head comparison of the two most widely studied mental health interventions for autistic adults: cognitive-behavioral therapy (CBT) and mindfulness-based therapy (MBT). Both interventions are well-established, empirically supported treatments for depression and anxiety in the general population, and both interventions have demonstrated efficacy among autistic adults. However, their comparative effectiveness and heterogeneity of treatment effects have not been established in autistic adults. Both interventions will be delivered by telehealth.