Co-occurring mental health difficulties among autistic adults are common and this association has implications for mental health services. Gaining a comprehensive understanding of autistic adults' experiences of accessing and receiving mental health support is a critical step to fully inform service provision. We conducted a systematic review and meta-synthesis of qualitative studies exploring autistic adults' mental health service experiences, triangulating the perspectives of autistic adults, clinicians and parents. Electronic database searches in MEDLINE, PsycINFO and Embase were conducted up to October 2021. This identified 10,068 studies, of which 38 met study inclusion criteria. Using a thematic synthesis approach, we generated three superordinate analytical themes "Lonely, difficult service experience", "Complexity needs flexibility" and "Collaboration and empowerment", each with several subthemes. Our findings show that currently, mental health services do not adequately support autistic adults, and can even cause additional harm. There is a need for a more flexible, comprehensive and holistic approach, considering how being autistic affects the individual's mental health presentation and tailoring support to their needs. Building trusting relationships, listening to autistic adults, and empowering them to take agency, appear to be fundamental steps towards more successful mental health care provision.