Despite repeated efforts by the Nigerian government and the international community, under-5 child mortality remains alarmingly high in Nigeria. This narrative review aims to reassess the key factors contributing to this persistent public health challenge. A comprehensive search of peer-reviewed articles and reports published in English was conducted to identify and synthesize data on the factors predisposing Nigerian children under 5 to mortality. The review identifies multiple interrelated contributors, including socioeconomic, sociocultural, and demographic factors, inadequate access to healthcare services, an under-resourced healthcare system, and a shortage of qualified healthcare professionals. The high burden of communicable and preventable diseases also plays a significant role in under-5 mortality. To address these issues, targeted interventions such as improving healthcare access, strengthening the health system, and reducing poverty are essential. The findings underscore the urgent need for a coordinated, multi-sectoral approach to effectively reduce under-5 mortality in Nigeria and improve the health outcomes of vulnerable children. Government, healthcare providers, and communities must work together to address these concerns so that all children can access the care they need to live and flourish.