Autistic adults commonly experience anxiety and depression. These mental health concerns are often tied to social experiences, such that mental well-being can be supported by social connection and deteriorated by loneliness. The mediating role of social and emotional loneliness (i.e. social isolation and lack of emotional attachment, respectively) between autism features and mental health has yet to be empirically tested among autistic adults. Here, 69 autistic young adults completed self-report questionnaires assessing social contact (Friendship Questionnaire), autism features (Autism Quotient), mental health (Liebowitz Social Anxiety Scale, Social Phobia Inventory, Beck Depression Inventory), and loneliness (Social and Emotional Loneliness Scale for Adults). Positive associations emerged between autism features, social loneliness, family loneliness, social anxiety, and depression. In addition, more social contact was related to less social and family loneliness and less social anxiety but was not related to depression. Mediation analyses indicated significant indirect effects of social contact and autism features on mental health through social loneliness. Indirect effects partially held substituting family loneliness for social loneliness and did not hold using romantic loneliness. In light of these results, the scientific and clinical implications of the role of loneliness for autistic young adults are discussed and recommendations provided.