Abstract: The Coronavirus Disease (COVID-19) epidemic was first detected in China in December 2019 and spread to other countries quickly. Many studies have shown that the COVID-19 pandemic adversely affected mental health. Individual differences such as personality could influence people’s responses to the pandemic. In our first study, we examined how personality traits of Neuroticism and Extraversion (using the Five-Factor Model as our framework) were related to the mental health of Canadians during the first year of the COVID-19 pandemic. Using data from an online survey with 1096 responses and controlling for demographic variables, we found that personality traits of Neuroticism and Extraversion are associated with the current mental health of Canadians during the COVID-19 pandemic, with Extraversion positively related to mental health and Neuroticism negatively related to it. In our second study, we tried to replicate the results of our first study, a year later, in the second year of the pandemic. Our results were similar to our first study and we found that Extraversion was positively and Neuroticism negatively was associated with the mental health of Canadians. We also extended our first study by including all 5 personality traits in our analysis and controlling for coping and response to COVID-19 stress (in addition to demographic variables) and found that Agreeableness, Openness to Experience, and Conscientiousness were positively and significantly related to the mental health of Canadians. In our third study, we examined how personality traits (using the Five-Factor Model as our framework and controlling for coping and response to COVID-19 stress and demographic variables) and coping and response to COVID-19 stress were related to the mental health of Canadian children and adolescents during the second year of the pandemic. Using parent reports of 100 preschoolers and 607 7–18-year-old children, we found that the big 5 personality traits were associated with the mental health of Canadian youth during COVID-19 pandemic. In preschoolers, Neuroticism and Agreeableness predicted the most mental health problems, and in 6-18-year-old children, Extraversion negatively predicted the most mental health problems. Also, Openness to Experience was the weakest predictor of mental health status in Canadian youth. Furthermore, we conducted a moderated hierarchical regression analysis to determine whether the relationship between personality traits and SDQ variables is moderated by age in 7–18-year-old children. Our results indicated that age did not moderate the relationship between personality traits and SDQ variables with the exception of a negative interaction effect between age and Extraversion on Hyperactivity/Inattention and a very weak positive interaction effect between age and Conscientiousness on Total Difficulty score. The results of this study could have important implications for mental health research, prevention, and treatment of mental health issues in adults and children. For instance, since the results of our study showed that different individuals with different personality traits are experiencing the pandemic differently, thus, they may need mental health services that are more specifically tailored to their personality type. Also, our findings could help healthcare providers to develop more targeted interventions for prevention or treatment. Moreover, by identifying personality traits that could put children at risk for mental health problems, healthcare professionals and caregivers can take steps to intervene early and prevent the development of more severe mental health problems and also tailor treatment plans for children with mental health issues.