Job stress and death from overwork (Karoshi) are serious health concerns in Japan. This observational study examined the relationship between baseline mental health promotion behaviors (MHPBs; physical activity, weekday cultural activities, enriching activities during vacations, interpersonal communication, relaxation, volunteering, interest and participation in new activities, and group affiliation) and condition of job-related mood (vigor, anger–irritability, fatigue, anxiety, and depression) at a 6-month follow-up to promote the resolution of mental health problems among Japanese employees of small- to medium-sized enterprises. Baseline and follow-up surveys were conducted with N = 635 participants. Participants reported their current engagement in MHPBs and current level of job-related mood as vigor (a positive mood state in the form of energy and enthusiasm for the job), anger–irritability, fatigue, anxiety, and depression. Structural equation modeling showed MHPBs to predict vigor, fatigue, and depression at both baseline and follow-up. Baseline MHPBs and job-related mood were carried over to a 6-month follow-up. Baseline job-related mood was found to mediate the relationship between baseline MHPBs and follow-up job-related mood. These findings suggest that engagement in MHPBs has contributed to improving job-related mood, especially vigor in Japanese workers, and may be applicable in other international settings.