Background <p>This article examines the association between population density, maternal mortality, and under-5 mortality in countries throughout the world, as well as the mediating impacts of the Universal Health Coverage Service Coverage Index (UHC-SCI).</p> Methods <p>The World Health Organization’s website provided data on maternal mortality and the Universal Health Coverage Service Coverage Index for the years 2000–2020. The World Bank database included information on population density and under-5 mortality rates for nations between 2000 and 2020. Panel regressions were used to examine the association between population density and maternal and under-5 mortality in each nation, as well as the mediating influence of the Universal Health Coverage Service Coverage Index, while accounting for economic, environmental, and medical factors. Finally, data is divided into regressions based on World Bank member countries’ income levels to examine heterogeneity.</p> Results <p>The study included 175 countries and found a significant negative correlation between population density, maternal mortality, and under-5 mortality (<i>B</i> = -1.015, -1.146, <i>P</i> &lt; 0.05). The Universal Health Coverage Service Coverage Index mediated this relationship (<i>B</i> = -1.044, -1.141, <i>P</i> &lt; 0.05).</p> Conclusions <p>Increasing population density in countries around the world has helped to reduce maternal and child mortality. As population density has increased, so has the level of the Universal Health Coverage Service Coverage Index, which has proven effective in lowering maternal and under-5 mortality. Governments should plan interventions to build basic health facilities and allocate resources to health services based on population density, level of economic development, and the current state of their health systems, with the goal of stabilizing the rate of change in maternal and under-5 mortality and, eventually, achieving the Sustainable Development Goals.</p>