COVID-19 has had a negative impact on the mental health of individuals. The aim of the COVID-19 Psychological Wellbeing Study was to identify trajectories of anxiety, depression and COVID-19-related traumatic stress (CV19TS) symptomology during the first UK national lockdown. We also sought to explore risk and protective factors. The study was a longitudinal, three-wave survey of UK adults conducted online. Analysis used growth mixture modelling and logistic regressions. Data was collected from 1958 adults. A robust 4-class model for anxiety, depression, and CV19TS symptomology distinguished participants in relation to the severity and stability of symptomology. Classes described low and stable and high and stable symptomology, and symptomology that improved or declined across the study period. Several risk and protection factors were identified as predicting membership of classes (e.g., mental health factors, sociodemographic factors and COVID-19 worries). This study reports trajectories describing a differential impact of COVID-19 on the mental health of UK adults. Some adults experienced psychological distress throughout, some were more vulnerable in the early weeks, and for others vulnerability was delayed. These findings emphasise the need for appropriate mental health support interventions to promote improved outcomes in the COVID-19 recovery phase and future pandemics.