Small island developing states face particular challenges to ensure their infrastructure promotes social, economic, and environmental well-being. Closing the achievement gap for the Sustainable Development Goals (SDGs) involves prioritization and coordination across multiple sectors. With an application to the country of Saint Lucia, this paper develops a stakeholder-driven analysis framework integrating four interdependent infrastructure sectors (electricity, water, wastewater, and solid waste). Drawing on extensive consultation with decision-makers in-country—134 stakeholders from 18 government ministries, agencies, academia, or the private sector—the analysis identifies specific interventions that could be implemented over the next decades to meet future needs for sustainable infrastructure services. These interventions are congruent with the government's development plans. Long-term, cross-sectoral portfolios of investments and policies (“strategies”) are developed which demonstrably reach the targets of the SDGs and Saint Lucia's emissions reduction commitments under the Paris Agreement. The sequencing of these investments or policies is designed to optimize their efficiency and impact over time, identifying “quick wins” while ensuring that there is sufficient action to provide services in the long-term sustainably. A comparison of costs associated with each strategy suggests that accounting for interdependencies and taking a long-term perspective can save costs over the life of infrastructure investments. This process of infrastructure assessment is applicable beyond the small island context, allowing practitioners a means to undertake systematic assessment of a country's future infrastructure needs and to develop appropriate solutions aligned with its national objectives and international commitments.