Anxiety in autism is an important treatment target because of its consequences for quality of life and well-being. Growing evidence suggests that cognitive behaviour therapies and mindfulness-based therapies can ameliorate anxiety in autism but cost-effective delivery remains a challenge. This pilot randomised controlled trial examined whether online cognitive behaviour therapy and mindfulness-based therapy self-help programmes could help reduce anxiety in 54 autistic adults who were randomly allocated to either an online cognitive behaviour therapy (n = 16) or mindfulness-based therapy (n = 19) programme or a waitlist control group (n = 19). Primary outcome measures of anxiety, secondary outcome measures of broader well-being and potential process of change variables were collected at baseline, after programme completion, and then 3 and 6 months post-completion. Baseline data confirmed that intolerance of uncertainty and emotional acceptance accounted for up to 61% of self-reported anxiety across all participants. The 23 participants who were retained in the active conditions (14 mindfulness-based therapies, 9 cognitive behaviour therapies) showed significant decreases in anxiety that were maintained over 3, and to some extent also 6 months. Overall, results suggest that online self-help cognitive behaviour therapy and mindfulness-based therapy tools may provide a cost-effective method for delivering mental health support to those autistic adults who can engage effectively with online support tools.