BACKGROUND The effects of the Covid-19 pandemic on mental health remain unclear. To mitigate the risks and capitalise on opportunities for positive change, we must understand how the impact has been mediated by sociodemographics, mental disorders, personality HP traits, life circumstances and the coping measures people choose to take. METHODS Data were collected from 376,987 members of the general public, predominantly in the UK, between late December 2019 and May 2020. Interaction models examined differences in sociodemographic distributions of mood and anxiety HP for 233,268 people in January vs. 109,749 in May. Factor analysis of a comprehensive instrument determined the dimensionality of self-perceived pandemic-driven change in wellbeing, outlook and behaviour for 74,830 participants in May. Linear modelling identified demographic, contextual, clinical, and trait predictors of pandemic impact. Topic modelling distilled prevalent advice from free-text responses. RESULTS Anxiety HP, depression and insomnia HP changed markedly in demographically-mediated ways. Untoward changes were larger for older adults TRANS. Benefits were greater for younger adults TRANS. Social connectedness was negatively affected across most mental and neurological conditions. There were disorder-specific changes in other domains, e.g., heightened conflict at home for attention-deficit hyperactivity disorder HP and heightened anxiety HP for obsessive-compulsive disorder. Psychiatric symptoms, personality traits, occupational variables and living conditions were amongst the strongest predictors of pandemic impact. Frontline health workers, carers of vulnerable older adults TRANS, and disabled or sheltered adults TRANS were disproportionately affected. Fifty advice topics were identified from free-text, the prevalence SERO of which covaried with subpopulation, context and traits. CONCLUSIONS The general public report positive and negative consequences of the pandemic. Particular subsets of people have heightened risk of untoward effects whereas other groups appear resilient. To be valid and effective, studies seeking to quantify, predict or mitigate the impact of pandemics on mental health should apply holistic approaches, combining multiple psycho-socio-economic factors.