The business environment in Nigeria is characterised by significant challenges that impede enterprise growth and financial inclusion. A comprehensive, integrated analysis of the interplay between governance, infrastructure, and financial access is required to inform effective policy. This study aims to diagnose the systemic constraints within the Nigerian business ecosystem by examining the relationships between institutional governance, physical infrastructure quality, and access to formal and informal finance for micro, small, and medium-sized enterprises (MSMEs). A sequential explanatory mixed-methods design was employed. Quantitative analysis of enterprise-level panel data established correlational patterns, followed by in-depth qualitative interviews with entrepreneurs and key institutional stakeholders to explicate causal mechanisms and lived experiences. Quantitative analysis revealed that firms reporting 'major' or 'severe' infrastructure constraints were 40% less likely to secure formal credit. Thematic analysis of interview data identified a pervasive 'informal adaptation' strategy, where entrepreneurs bypass failing formal systems but incur high transaction costs. The findings demonstrate a self-reinforcing cycle where poor governance undermines infrastructure, which in turn restricts financial access, forcing enterprises into inefficient informal arrangements that limit scalability. Policy must move beyond siloed interventions. We recommend integrated 'business environment compacts' that link targeted infrastructure investment in economic clusters with parallel reforms to local regulatory enforcement and the development of asset-based lending products. Business environment, mixed methods, financial access, infrastructure, governance, MSMEs, Nigeria This study provides a novel integrated diagnostic framework and a unique merged dataset, demonstrating the critical interdependencies between governance, infrastructure, and finance that are typically analysed in isolation.