Autistic children grow to become autistic adults, and autism is increasingly diagnosed in adulthood and later life. This qualitative study aimed to understand experiences of autism throughout adulthood. A national cohort study of autistic adults and relatives of autistic adults (ASC-UK), enabled purposive recruitment of a diverse sample. Semi-structured interviews were conducted with 29 autistic adults (aged 20–71 years), mostly diagnosed in adulthood, and 16 relatives (aged 31–81 years) of autistic adults diagnosed across both childhood and adulthood (including some with learning disability). Interview topics included health, relationships, education, employment, quality of life and everyday experiences. Thematic analysis of the accounts of the autistic adults identified six key themes relating to their experiences: (1) diagnosis as validating yet limiting; (2) supportive and non-supportive social agents; (3) the “invisibility” of the needs of autistic adults; (4) health in the context of autism; (5) staying ‘outside’ the circle; and (6) multiple lives with autism. Data from relatives about autistic adult experiences gave additional perspectives on these themes. Experiences reported in other studies–of ‘difference’ from others, challenges of social engagement, and learning to ‘conform’ to society’s expectations–were evident and relevant to male and female autistic adults, across all age groups, and unrelated to stage of life when diagnosed. Some expressed disappointment with their lives, but others were proud of their achievements. Education and employment, whilst challenging for many, were also rewarding for some. Health care and social services were often experienced as inaccessible, inappropriate, or lacking understanding of the individual’s needs. We conclude that greater public understanding of autism as experienced in adulthood is needed. Key priorities are improving the availability of ‘appropriate’ health and social care services for autistic adults and families, and providing practical support to enable enhanced participation in life.