The COVID-19 pandemic is one of the most serious health and economic crises of the 21st century. From a psychological point of view, the COVID-19 pandemic and its consequences can be conceptualized as a multidimensional and potentially toxic stressor for mental health in the general population. This selective literature review provides an overview of longitudinal studies published until June 2021 that have investigated the impact of the COVID-19 pandemic on mental health in the European population. Risk and protective factors identified in the studies are summarized. Forty-two studies that met inclusion and search criteria (COVID-19, mental health, longitudinal, and Europe) in PubMed, PsycInfo, and Web of Science databases indicate differential effects of the pandemic on mental distress, depression, and anxiety, depending on samples and methods used. Age-specific (e.g., young age), social (e.g., female, ethnical minority, loneliness), as well as physical and mental health-related factors (e.g., pre-pandemic illness) were identified as risk factors for poor mental health. The studies point to several protective factors such as social support, higher cognitive ability, resilience, and self-efficacy. Increasing evidence supports the assumption of the pandemic being a multidimensional stressor on mental health, with some populations appearing more vulnerable than others, although inconsistencies arise. Whether the pandemic will lead to an increase in the prevalence of mental disorders is an open question. Further high-quality longitudinal and multi-national studies and meta-analyses are needed to draw the complete picture of the consequences of the pandemic on mental health.