Previous research shows that autistic people have high levels of co-occurring mental health conditions. Yet, a number of case reports have revealed that mental health conditions are often misdiagnosed in autistic individuals. A total of 420 adults who identified as autistic, possibly autistic or non-autistic completed an online survey consisting of questions regarding mental health diagnoses they received, whether they agreed with those diagnoses and if not why. Autistic and possibly autistic participants were more likely to report receiving mental health diagnoses compared to non-autistic participants, but were less likely to agree with those diagnoses. Thematic analysis revealed the participants' main reasons for disagreement were that (1) they felt their autism characteristics were being confused with mental health conditions by healthcare professionals and (2) they perceived their own mental health difficulties to be resultant of ASC. Participants attributed these to the clinical barriers they experienced, including healthcare professionals' lack of autism awareness and lack of communication, which in turn prevented them from receiving appropriate support. This study highlights the need for autism awareness training for healthcare professionals and the need to develop tools and interventions to accurately diagnose and effectively treat mental health conditions in autistic individuals.