Limiting global warming to 1.5°C requires leaving fossil fuels underground (LFFU), generating stranded resources and assets, and intensifying long-standing North-South injustices in climate governance. Since the implications for fossil-dependent low- and middle-income countries (LMICs) remain largely overlooked, this thesis asks: what political and institutional dilemmas LMICs face in their energy transitions and how can these be addressed. Using mixed methods and combining institutional analysis, a theory of change for inclusive development, a critical international political economy perspective and comparative case studies of Argentina, Brazil, and Colombia, this thesis compiling five published and in review papers concludes: LFFU in LMICs is a triple transition – energy, productive and just – in which two dilemmas emerge: the cost of action versus the cost of inaction and the feasibility of leapfrogging. Debt, high capital costs and fossil rents intensify these dilemmas toward inaction, leading to energy addition, hedging strategies, and rising stranded-asset exposure. Fossil fuel expansion and carbon lock-in are sustained by a historical bloc linking states, industry, and investors, which mobilises the Right to Development and fossil-fuel narratives to legitimise continued extraction.Counter-hegemonic coalitions can reduce stranded-asset exposure and enable energy leapfrogging by closing the extractive frontier, sequencing productive diversification and just transition policies, reshaping institutions and development imaginaries. Treating LFFU both as a technical challenge and a political project can accelerate qualitative leapfrogging trajectories and reduce the costs of inaction, avoiding the irreversible consequences of climate change. By foregrounding power, justice, narratives, and governance pathways, this thesis advances a political economy perspective on just and inclusive energy transitions in the Global South.