Introduction: Vast available international evidence has investigated the mental health impacts of the COVID-19 pandemic. This review aims to synthesise evidence, identifying populations and characteristics associated with poor mental health.Methods: A meta-review of pooled prevalence of anxiety and depression, with subgroup analyses for the general population, healthcare workers (HCW) and COVID-19 patients; and a meta-synthesis of systematic reviews to collate evidence on associated factors and further mental disorders. Databases searched included Scopus, Embase, PsycINFO, and MEDLINE dated to May 2021. Eligibility criteria included systematic reviews and/or meta-analyses, published post-November 2019, reporting data in English on mental health outcomes during the pandemic.Results: Eighty-one systematic reviews were included, 51 of which incorporated meta-analysis. Meta-review overall anxiety prevalence was 29% (95%CI: 27–31%, I2: 99.83%), with subgroup prevalence as 35% (95%CI: 23–47%, I2: 97.4%) in COVID-19 patients, 29% in HCW (95%CI: 25– 32, I2: 99.8%) and 28% in the general population (95%CI: 25–31%, I2: 99.9%). Meta-review overall depression prevalence was 28% (95%CI: 26–30%, I2: 99.7), with subgroup prevalence as 30% (95%CI: 7–60%, I2: 99.8%) in COVID-19 patients, 28% (95%CI: 25–31%, I2: 99.7%) in HCW and 27% (95%CI: 25–30, I2: 99.8%) in the general population. Meta-synthesis found many experienced psychological distress and PTSD/PTSS during COVID-19, but pooled prevalence ranged substantially. Fear of, proximity to, or confirmed COVID-19 infection; undergoing quarantine; and COVID-19-related news exposure were associated with adverse mental health outcomes. Amongst other factors, people who are younger, female, LGBTIQ, pregnant, parents or experiencing low social support, financial issues or socio-economic disadvantage, tended to have poorer mental health during the pandemic period.Conclusions: Despite high volumes of reviews, the diversity of findings and dearth of longitudinal studies within reviews means clear links between COVID-19 and mental health are not available, although existing evidence indicates probable associations.