The current study explored the role of pre-existing and pandemic-time child, family or environmental factors in the presentation of mental health symptoms of autistic youth and their parents during the pandemic. Participants were parents/carers of autistic children (Autism Spectrum Treatment and Resilience Cohort, N = 67, Mage = 9 years) and adolescents (QUEST cohort, N = 112, Mage = 17 years). Parents completed an online survey that asked about child and parental mental health, infection experience, and changes to education arrangements, family life, housing and finances during the pandemic. Pre-existing measures of mental health, autism and adaptive functioning were also utilised. More engagement and enjoyment in education provision and going outside was associated with better child and parental mental health. In multivariate multiple linear regression models, more pre-existing attention deficit hyperactivity disorder symptoms were associated with more behavioural/attention deficit hyperactivity disorder symptoms during the pandemic in the pre-adolescent cohort, and with greater emotional symptoms in the adolescent cohort. More pre-existing parental mental health problems were associated with more parental mental health symptoms during the pandemic in both cohorts. Knowledge of pre-existing mental health and pandemic-related stressors may help care planning. Encouraging engagement and enjoyment in education and promoting physical exercise are key intervention targets. Ensuring access to attention deficit hyperactivity disorder medication and support is important, especially if this is managed jointly across school and home. Lay abstract What is already known about the topic: The COVID-19 pandemic and the associated restrictions impacted all of society. There is emerging evidence showing a range of impacts on autistic children and young people and their families. Further research that looks at how individuals coped during the pandemic while considering how they were doing before the pandemic is needed. What this paper adds: This article explores whether how well autistic youth were doing before the pandemic influenced how they coped during the pandemic. It also looked at how well their parents were doing during the pandemic and whether any pre-pandemic factors influenced how they coped. Samples of both primary-school-aged autistic children and autistic teenagers and their parents were surveyed to answer these questions. More engagement and enjoyment in education provision during the pandemic and getting outside more were linked with better child and parental mental health during the pandemic. More attention deficit hyperactivity disorder before the pandemic was linked with more attention deficit hyperactivity disorder and behavioural problems during the pandemic in primary-school-aged autistic children, and more emotional problems during the pandemic in autistic teenagers. Parents with more mental health problems during the pandemic had more mental health problems before the pandemic. Implications for practice, research or policy: Encouraging engagement and enjoyment in education and promoting physical exercise are key intervention targets. Ensuring access to attention deficit hyperactivity disorder medication and support is important, especially if this is managed jointly across school and home.