An action checklist for improving the workplace environment by means of enhancing mental health of workers (Mental Health Action Check List: MHACL) was developed. The use of the checklist for primary prevention was examined. MHACL was developed through three steps: (1) Review of related references and collection of improvement examples for designing a draft MHACL; (2) pilot application of the draft at industrial workplaces and trials at workshops of occupational health staff; and (3) proposing a new MHACL for general use in industry. Workplace improvement actions related to mental health were listed in eight technical areas. From 84 workplaces in Japan, 201 such actions were collected. Typical improvement action phrases were extracted based on these examples, and a draft MHACL containing 40 generally applicable actions were prepared. This draft was applied to selected workplaces for its use as a tool for group discussion. Then, the utility of the checklist was discussed by 105 occupational health staff working in public service offices. The workshop suggested modifications of the draft MHACL including improved check items and usage procedures and the need to use easy-to-understand actions. The final version of the MHACL comprised 30 items in six technical areas: A) sharing work planning, B) work time and organization, C) ergonomic work methods, D) workplace environment, E) mutual support in the workplace, and F) preparedness and care. A new action checklist was proposed for use as a means of changing existing workplace environments and proposing practical actions for improving it. The checklist was confirmed to be useful for organizing workplace-level discussion for identifying immediate improvements at the workplace. The checklist is expected to be widely applied for promoting primary prevention measures in terms of better mental health.