The focus of this paper is sustainability assessment and planning of megacities of Global South (GS) and proposes a framework using the Federal Republic of Nigeria as a case study. The paper undertakes a systematic review of the mega and sustainable cities literature. Achieving inclusive, recreating and sustainable cities for all in the context of supreme rates of urbanisation, economy-oriented priorities, and chaotic urban development that characterise the developing world is a major challenge. As the world rapidly urbanises, there is much focus on achieving sustainability outcomes within cities. The analysis highlights a prospect in the reviewed academic literature for cities to become sustainable first to be considered truthfully smart. Accomplishing this goal requires not only envisioning sustainable cities and implementing strategies but also assessing progress toward sustainable urban development. Global population growth has resulted in increasing pressure on infrastructures (transportation, housing, water, electricity, etc.) with a noted negative impact on the environment of megacities and much so in the GS with limited systems and plans to support the growth. The primary goal of this paper is to develop an integrated model to analyse and track SDGs progress with megacities planning as key indicators. With the emergence of megacities in the 21st century and their continuous growth in population and economic power, the environmental impact has reached the global scale. This viewpoint brings a pressing reality to the need to build tomorrow's world on sustainability principles. The inherent limitations imposed in the GS further imply that achieving sustainable development goals 13 (SDG 11) demands measuring progress and development. This paper finds that urban sustainability valuation overall lacks a unifying framing and that it could be better aligned with common sustainability principles. It presents findings from the first-ever literature review on the carbon-neutral city concept, covering definitions, assessment approaches, and barriers and drivers for the transition to carbon neutrality. The paper offers recommendations for future urban sustainability valuation research, including the employment of mixed-methods research among other strategies.