This paper examines the influence of work-related stress, social support, fear of COVID-19, and demographics in promoting mental health (MH) amongst healthcare workers (HCWs) in Nigeria. Hence, it adopted a survey research design. The results showed that work-related stress, social support, fear of COVID-19, and demographics strongly and jointly influence healthcare workers’ mental health in Nigeria. Also, they indicated a significant independent influence of these independent factors on healthcare workers’ mental health. The results show that out of the demographics considered in this study (such as gender, age, marital status, level of education, profession, department, work experience, and state of residence), only marital status did not significantly influence the healthcare workers’ mental health in Nigeria. So, the government, health faculties, clinical psychologists, human resources managers, and medical practitioners should encourage reducing work-related stress. This should be done by increasing social support, reducing fear of COVID-19, and considering demographics while trying to promote healthcare workers’ mental health in Nigeria, especially during the current COVID -19 pandemic era. Thus, this paper has recognized work-related stress, social support, fear of COVID-19, and demographics as significant influencers in promoting mental health amongst healthcare workers in Nigeria.