The entrepreneurial ecosystem in sub-Saharan Africa is complex and under-researched, with a significant gap in understanding the longitudinal drivers of business viability. Tanzania presents a critical case study due to its evolving economic policies and diverse enterprise landscape. This study aims to identify and analyse the key institutional, behavioural, and financial factors that determine the survival and growth trajectories of small and medium-sized enterprises within the Tanzanian context. A sequential explanatory mixed-methods design was employed. A quantitative panel dataset of enterprise performance metrics was analysed using survival analysis and growth modelling. This was followed by in-depth qualitative interviews with founders and stakeholders to contextualise the statistical relationships. Quantitatively, enterprises with formalised bookkeeping practices demonstrated a 40% higher survival probability beyond the five-year mark. Qualitatively, a central theme was the strategic navigation of informal social networks to mitigate institutional voids, often prioritised over formal financing. Business viability is predominantly contingent on behavioural adaptations to the institutional environment, with financial management formalisation being a significant, yet insufficient, predictor of sustained growth. Policymakers should design support mechanisms that integrate formal financial literacy programmes with recognition of the value inherent in existing informal entrepreneurial networks. Financial institutions ought to develop credit instruments that accommodate non-standard business records. entrepreneurial finance, institutional voids, business survival, mixed methods, East Africa, SME growth This paper provides a novel, integrated behavioural-institutional framework for analysing business ecosystems in emerging markets, underpinned by a unique longitudinal dataset linking firm-level strategies with macroeconomic shifts.