Autistic people experience high rates of mental health problems. A tendency towards behavioural repetition in autistic individuals could contribute to repetitive negative thinking, which is a transdiagnostic risk factor for mental health problems. Our research aimed to add to the literature by using transdiagnostic measures of both mental health symptoms and repetitive negative thinking. We explored the relationships between insistence on sameness and depression-anxiety, via the mediating role of repetitive negative thinking, operationalised as rumination and obsessing. Three groups of adults were recruited; autistic clinical cohort participants (n = 67); autistic community participants (n = 54) and non-autistic community participants (n = 66). All participants completed measures of Insistence on sameness; Anxiety and Depression; Obsessing; Rumination. Autistic community participants had significantly higher insistence on sameness, rumination and obsessing scores than non-autistic community participants. The mediation analysis found support for the hypothesis that the relationship between insistence on sameness and depression-anxiety would be mediated by rumination and obsessing in all participants. The higher rates of repetitive negative thinking found in autistic individuals may contribute to elevated rates of mental health problems in this group. Mental health assessments and interventions for autistic people should consider transdiagnostic factors including multiple forms of repetitive negative cognition and behaviour.