Abstract Background We investigated the impact of the COVID-19 crisis on mental health of professionals working in the intensive care unit (ICU) according to the intensity of the epidemic in France. Methods This cross-sectional survey was conducted in 77 French hospitals from April 22 to May 13 2020. All ICU frontline healthcare workers were eligible. The primary endpoint was the mental health, assessed using the 12-item General Health Questionnaire. Sources of stress during the crisis were assessed using the Perceived Stressors in Intensive Care Units (PS-ICU) scale. Epidemic intensity was defined as high or low for each region based on publicly available data from Santé Publique France. Effects were assessed using linear mixed models, moderation and mediation analyses. Results In total, 2643 health professionals participated; 64.36% in high-intensity zones. Professionals in areas with greater epidemic intensity were at higher risk of mental health issues (p < 0.001), and higher levels of overall perceived stress (p < 0.001), compared to low-intensity zones. Factors associated with higher overall perceived stress were female sex (B = 0.13; 95% confidence interval [CI] = 0.08–0.17), having a relative at risk of COVID-19 (B = 0.14; 95%-CI = 0.09–0.18) and working in high-intensity zones (B = 0.11; 95%-CI = 0.02–0.20). Perceived stress mediated the impact of the crisis context on mental health (B = 0.23, 95%-CI = 0.05, 0.41) and the impact of stress on mental health was moderated by positive thinking, b = − 0.32, 95% CI = − 0.54, − 0.11. Conclusion COVID-19 negatively impacted the mental health of ICU professionals. Professionals working in zones where the epidemic was of high intensity were significantly more affected, with higher levels of perceived stress. This study is supported by a grant from the French Ministry of Health (PHRC-COVID 2020).