Aim The Mental Health Supporter Training Program is a national project conducted in Japan. This study aimed to determine the effects on mental health‐related stigma, mental health literacy, and knowledge about mental health difficulties and support techniques among program participants. Methods The target population was local residents of a wide range of generations in Japan. Outcomes were assessed at baseline (T1), immediately postintervention (T2), and at the 6‐month follow‐up (T3). A mixed model for repeated‐measures conditional growth model analyses were employed to examine the effects of the intervention over time (T1, T2, T3). We also calculated effect sizes using Cohen's d. Results The program had a significantly favorable pooled effect on the Japanese version of the Reported and Intended Behaviour Scale score after adjusting for covariates (reported behavior t = 3.20, p = 0.001; intended behavior t = 8.04, p < 0.001). However, when compared at each time point, only intended behavior from T1 to T2 showed a significant difference (t = 8.37, p < 0.001). Significant pooled effects were found for mental health literacy (knowledge: t = 19.85, p < 0.001; attitude: t = 15.02, p < 0.001), knowledge of mental health (t = 28.04, p < 0.001), and psychological distress (t = −2.41, p = 0.016). Conclusion The results suggest that the program might be effective for improving intended, but not reported, behavior in the short term and for improving mental health literacy, knowledge of mental health, and psychological distress.