Understanding the impact of COVID-19 on different population cohorts and which personality traits affected individual's coping responses can help identify strategies to promote self-directed behaviours, thereby enhancing and maintaining individual's mental well-being.Using longitudinal data for the UK, we examine the impact of the COVID-19 pandemic on individuals' mental well-being, focusing on age, gender, and personality traits as possible modifiers.We explore the longitudinal nature of the data using individual fixed effects models, which implicitly control for unobserved time-invariant individual-level characteristics. Our sample is an unbalanced panel consisting of 373,555 person-years observations, observed from 2009 until June 2020.The negative impacts of the first months of the pandemic period are found to be larger for young adults (aged 16-25 years) and vary by personality traits. The increase in psychological distress symptoms is more pronounced for individuals who score higher in neuroticism, extroversion, and openness to experience. Indeed, for introverted young people, recent events may have actually brought a sense of calm. Other findings indicate that worsening in the psychological distress level occurs alongside with increased feelings of loneliness.Our findings support the theoretical knowledge that different people have different psychological and behaviour responses and personality concepts can be used when studying individual's adaptive behaviour in critical situations such as COVID-19. Our results indicate the necessity of public health programmes to assist distressed young individuals.