There is increasing evidence of the psychological impact of COVID-19 on various population groups, with concern particularly focused on young people’s mental health. However, few papers have engaged with the views of young people themselves. We present findings from a study into young people’s discussions on social media about the impact of COVID-19 on their mental health. Real-time, multi-platform online ethnography was used to collect social media posts by young people in the United Kingdom (UK), March 2020–March 2021, 1033 original posts and 13,860 associated comments were analysed thematically. Mental health difficulties that were described as arising from, or exacerbated by, school closures, lost opportunities or fraught family environments included depression, anxiety and suicidality. Yet, some also described improvements to their mental health, away from prior stressors, such as school. Young people also recounted anxiety at the ramifications of the virus on others. The complexities of the psychological impact of COVID-19 on young people, and how this impact is situated in their pre-existing social worlds, need recognising. Forging appropriate support necessitates looking beyond an individualised conceptualisation of young people’s mental health that sets this apart from broader societal concerns. Instead, both research and practice need to take a systemic approach, recognising young people’s societal belonging and social contexts.