Objective: To examine the associations between mental health and lifestyle in adults with type 1 and type 2 diabetes mellitus (T1DM and T2DM). Methods: Online survey data from the cross-sectional Diabetes MILES – The Netherlands Study was analysed, including 270 adults with T1DM and 325 with T2DM. Mental health status (flourishing, moderate and languishing) in relation to diet, physical activity, alcohol consumption and smoking was analysed with ANCOVA and logistic regressions (adjusted for confounders). Results: 47% of T1DM-, and 55% of T2DM participants reported flourishing mental health. Due to an insufficient number, participants with languishing mental health were excluded. In T2DM, participants with flourishing mental health had more optimal diet quality (mean ± SEM: 70 ± 1 vs 68 ± 1 diet quality score, p = 0.015), and physical activity levels (mean ± SEM: 3484 ± 269 vs 2404 ± 273 MET minutes/week, p = 0.001) than those with moderate mental health, but did not differ with respect to alcohol consumption and smoking. In T1DM, no significant associations were found. Conclusion: Only in T2DM, people with flourishing mental health had more optimal lifestyle behaviours compared to people with moderate mental health. Further research is needed to determine if mental health is more important for specific lifestyle behaviours, and if the mental health effect differs across diabetes types.