Introduction – Alerted by the scientific community to the potential impact of the COVID-19 pandemic on the mental health of the population, Santé publique France established a surveillance system in March 2020. From a crisis management perspective, the objective was to collect the information necessary to implement measures to prevent and protect the population. Methods – Santé publique France’s surveillance system relied on the analysis of medical diagnoses reported by healthcare professionals (Health Surveillance of Emergency Room Visits and Deaths – Sursaud® –; National Program for the Medicalization of Information Systems in Medicine, Surgery, Obstetrics, and Dentistry – PMSI-MCO –) and on self-report surveys conducted among population samples (n=4) over a period from March 2020 to January 2022. The main indicators monitored were mood disorders, anxiety disorders, and suicidal behavior. Results – The surveillance systems primarily recorded an increase in depressive and anxiety symptoms among adults (affecting more than 20% of the population during lockdown periods) and emergency department visits for mood disorders, suicidal acts, and thoughts (with increases of up to nearly 200%) among adolescents and young adults. Inequalities were observed based on housing conditions, employment status, financial situation, personal history of psychological disorders, living alone, or having experienced COVID-19 symptoms. The most striking finding concerns age-related effects, with more negative and longer-lasting impacts observed among adolescents and young adults. Conclusions – The findings of the studies confirmed the negative impact of the health crisis and restrictive measures on the population’s mental health. It also highlighted the importance of having responsive and regular mental health monitoring systems in crisis situations. These systems have been essential tools for guiding the preventive response and advocating for the strengthening of mental health prevention and promotion policies.