The COVID-19 pandemic and its policy responses have had a detrimental effect on millions of people's mental health. Here, we investigate its impact on autistic people and their families using qualitative methods. Specifically, we addressed: how did autistic people experience an increase in social isolation during the initial lockdown? And how was their mental health impacted by lockdown? Autistic and non-autistic researchers conducted 144 semi-structured interviews with autistic adults (n = 44), parents of autistic children (n = 84) including autistic parents and autistic young people (n = 16). We deployed thematic analysis to identify key themes. The enhanced social isolation accompanying the pandemic had a serious and damaging impact on autistic people's mental health and subjective wellbeing. They spoke of intensely missing friends and more incidental forms of social connection. They also reported intense dissatisfaction with the substitution of embodied, person-to-person connection in health services by online/telephone-based alternatives, sometimes accompanied by serious negative consequences. These findings reveal the fundamental importance of supporting autistic people to maintain direct and incidental social contact during the pandemic and beyond. They speak against established theories that downplay autistic people's need for human connection and the extent to which they have been affected by social isolation during lockdowns.