Unplanned urban growth has emerged as one of the most pressing and complex challenges facing developing countries in the 21st century. The rapid and often uncontrolled influx of people into urban areas—driven by rural-to-urban migration, population growth, and the concentration of economic opportunities—has far outpaced the institutional and infrastructural capacity of governments. As a result, cities in the Global South are increasingly characterized by informal settlements, slums, and peripheral urban sprawl, where access to basic public infrastructure is limited or altogether absent. This research paper provides a comprehensive analysis of the multifaceted impacts of unplanned urban expansion on critical infrastructure sectors, including transportation networks, water and sanitation systems, housing availability, healthcare services, and educational facilities. By examining urban centers such as Lagos (Nigeria), Dhaka (Bangladesh), and Nairobi (Kenya), the study identifies a clear and compelling link between the proliferation of unregulated settlements and the declining quality, efficiency, and equity of public services. Factors such as inadequate land management, weak urban planning institutions, political neglect, and insufficient investment contribute to the breakdown of urban systems. This in turn leads to congested roads, polluted environments, inadequate waste disposal, overcrowded schools and hospitals, and heightened vulnerability to public health crises and environmental hazards. The findings underscore the urgent need for a paradigm shift in urban governance and policy formulation. There is a critical imperative for cities in developing nations to adopt inclusive, participatory, and data-driven urban planning approaches. Integrated infrastructure development, regulatory reforms, and targeted investments in underserved areas are essential to address the adverse consequences of unplanned urban growth. Only through coordinated, sustainable, and people-centered urban management can developing countries build resilient cities that ensure social equity, environmental sustainability, and improved quality of life for all urban residents.