Opportunities for adults to access support following a clinical diagnosis of autism are limited. This study investigated perspectives of autistic adults, relatives and clinicians regarding characteristics of optimal adult autism post-diagnosis support and services. In Stage 1, key stakeholders were surveyed about recent experiences of UK adult autism post-diagnostic services immediately following assessment/up to 12 months afterwards. Information gathered from Stage 1 was used to devise a set of statements describing optimal post-autism diagnostic support/service characteristics. In Stage 2 statements were presented to clinicians in a modified Delphi process. Data analyses were non-parametric and descriptive. Three hundred forty-three autistic adults and 45 relatives completed the Stage 1 surveys. Thirty-five clinicians completed a parallel survey. Just over half of adults and relatives reported receiving a follow-up appointment. Fewer than 40% of autistic people received support/services 12 months after diagnosis. Eleven statements describing optimal post-autism diagnostic support/service provision were developed and consensus among clinicians was reached on all. Autistic adults, relatives and clinicians described some aspects of post-autism diagnosis support and services positively; however, there were significant opportunities for improvement. The study findings can be used to develop current UK post-diagnosis support and services and may be relevant internationally.