This study aimed to evaluate the impact of COVID-19 pandemic on mental well-being observed during the first quarantine implemented in France in 2020. This online survey included 1,876 French volunteer participants who completed data on lifestyle habits during the period of confinement and self-reported levels of resilience, optimism and psychological well-being. We observed that the score of participants’ psychological well-being was considerably lower among single participants, students, the unemployed and those facing risk of financial insecurity. The results of the multivariate analysis showed that low levels of psychological well-being were significantly associated with single status and male gender. Higher scores for general peer support, optimism, resilience and confidence in information shared within immediate circles of friends of family were also significantly positively associated with better mental health. Taken together, the results from this study showed that quarantine measures impacted the psychological well-being of the participants in our sample, that the degree of impact was strongly linked with individual levels of resilience and optimism, and that general support together with confidence in one's immediate circle of friends or family also played an important role in overall mental well-being.